All I did was put an "if (NOT TARGET dlib)" around the main body of the CMakeLists.txt

so that we avoid a common cmake warning.

dlib/CMakeLists.txt

@@ -41,40 +41,49 @@ if (NOT CMAKE_CXX_FLAGS_DEBUG MATCHES "-DENABLE_ASSERTS")
       FORCE)
 endif ()
 
-set (DLIB_ISO_CPP_ONLY_STR 
-"Enable this if you don't want to compile any non-ISO C++ code (i.e. you don't use any of the API Wrappers)" )
-set (DLIB_NO_GUI_SUPPORT_STR 
-"Enable this if you don't want to compile any of the dlib GUI code" )
-set (DLIB_ENABLE_STACK_TRACE_STR 
-"Enable this if you want to turn on the DLIB_STACK_TRACE macros" )
-set (DLIB_ENABLE_ASSERTS_STR 
-"Enable this if you want to turn on the DLIB_ASSERT macro" )
-set (DLIB_USE_BLAS_STR
-"Disable this if you don't want to use a BLAS library" )
-set (DLIB_USE_LAPACK_STR
-"Disable this if you don't want to use a LAPACK library" )
-set (DLIB_LINK_WITH_LIBPNG_STR
-"Disable this if you don't want to link against libpng" )
-set (DLIB_LINK_WITH_LIBJPEG_STR
-"Disable this if you don't want to link against libjpeg" )
-set (DLIB_LINK_WITH_SQLITE3_STR
-"Disable this if you don't want to link against sqlite3" )
-
-option(DLIB_ISO_CPP_ONLY ${DLIB_ISO_CPP_ONLY_STR} OFF)
-option(DLIB_NO_GUI_SUPPORT ${DLIB_NO_GUI_SUPPORT_STR} OFF)
-option(DLIB_ENABLE_STACK_TRACE ${DLIB_ENABLE_STACK_TRACE_STR} OFF)
-option(DLIB_ENABLE_ASSERTS ${DLIB_ENABLE_ASSERTS_STR} OFF)
-option(DLIB_USE_BLAS ${DLIB_USE_BLAS_STR} ON)
-option(DLIB_USE_LAPACK ${DLIB_USE_LAPACK_STR} ON)
-option(DLIB_LINK_WITH_LIBPNG ${DLIB_LINK_WITH_LIBPNG_STR} ON)
-option(DLIB_LINK_WITH_LIBJPEG ${DLIB_LINK_WITH_LIBJPEG_STR} ON)
-option(DLIB_LINK_WITH_SQLITE3 ${DLIB_LINK_WITH_SQLITE3_STR} ON)
-
-
-add_library(dlib STATIC all/source.cpp )
-
-
-if (NOT DLIB_ISO_CPP_ONLY)
+
+# Don't try to call add_library(dlib) and setup dlib's stuff if it has already
+# been done by some other part of the current cmake project.  We do this
+# because it avoids getting warnings/errors about cmake policy CMP0002.  This
+# happens when a project tries to call add_subdirectory() on dlib more than
+# once.  This most often happens when the top level of a project depends on two
+# or more other things which both depend on dlib. 
+if (NOT TARGET dlib)
+
+   set (DLIB_ISO_CPP_ONLY_STR 
+   "Enable this if you don't want to compile any non-ISO C++ code (i.e. you don't use any of the API Wrappers)" )
+   set (DLIB_NO_GUI_SUPPORT_STR 
+   "Enable this if you don't want to compile any of the dlib GUI code" )
+   set (DLIB_ENABLE_STACK_TRACE_STR 
+   "Enable this if you want to turn on the DLIB_STACK_TRACE macros" )
+   set (DLIB_ENABLE_ASSERTS_STR 
+   "Enable this if you want to turn on the DLIB_ASSERT macro" )
+   set (DLIB_USE_BLAS_STR
+   "Disable this if you don't want to use a BLAS library" )
+   set (DLIB_USE_LAPACK_STR
+   "Disable this if you don't want to use a LAPACK library" )
+   set (DLIB_LINK_WITH_LIBPNG_STR
+   "Disable this if you don't want to link against libpng" )
+   set (DLIB_LINK_WITH_LIBJPEG_STR
+   "Disable this if you don't want to link against libjpeg" )
+   set (DLIB_LINK_WITH_SQLITE3_STR
+   "Disable this if you don't want to link against sqlite3" )
+
+   option(DLIB_ISO_CPP_ONLY ${DLIB_ISO_CPP_ONLY_STR} OFF)
+   option(DLIB_NO_GUI_SUPPORT ${DLIB_NO_GUI_SUPPORT_STR} OFF)
+   option(DLIB_ENABLE_STACK_TRACE ${DLIB_ENABLE_STACK_TRACE_STR} OFF)
+   option(DLIB_ENABLE_ASSERTS ${DLIB_ENABLE_ASSERTS_STR} OFF)
+   option(DLIB_USE_BLAS ${DLIB_USE_BLAS_STR} ON)
+   option(DLIB_USE_LAPACK ${DLIB_USE_LAPACK_STR} ON)
+   option(DLIB_LINK_WITH_LIBPNG ${DLIB_LINK_WITH_LIBPNG_STR} ON)
+   option(DLIB_LINK_WITH_LIBJPEG ${DLIB_LINK_WITH_LIBJPEG_STR} ON)
+   option(DLIB_LINK_WITH_SQLITE3 ${DLIB_LINK_WITH_SQLITE3_STR} ON)
+
+
+   add_library(dlib STATIC all/source.cpp )
+
+
+   if (NOT DLIB_ISO_CPP_ONLY)
       # we want to link to the right stuff depending on our platform.  
       if (WIN32 AND NOT CYGWIN) ###############################################################################
          if (DLIB_NO_GUI_SUPPORT)
@@ -208,63 +217,65 @@ if (NOT DLIB_ISO_CPP_ONLY)
 
       target_link_libraries(dlib ${dlib_needed_libraries} )
 
-endif ()  ##### end of if NOT DLIB_ISO_CPP_ONLY ##########################################################
+   endif ()  ##### end of if NOT DLIB_ISO_CPP_ONLY ##########################################################
 
 
-#test for some things that really should be true about the compiler
-include(TestForSTDNamespace)
-include(TestForANSIStreamHeaders)
+   #test for some things that really should be true about the compiler
+   include(TestForSTDNamespace)
+   include(TestForANSIStreamHeaders)
 
 
-if (DLIB_LINK_WITH_LIBPNG AND NOT DLIB_ISO_CPP_ONLY)
+   if (DLIB_LINK_WITH_LIBPNG AND NOT DLIB_ISO_CPP_ONLY)
       add_global_define(DLIB_PNG_SUPPORT)
-else()
+   else()
       remove_global_define(DLIB_PNG_SUPPORT)
-endif()
+   endif()
 
-if (DLIB_LINK_WITH_LIBJPEG AND NOT DLIB_ISO_CPP_ONLY)
+   if (DLIB_LINK_WITH_LIBJPEG AND NOT DLIB_ISO_CPP_ONLY)
       add_global_define(DLIB_JPEG_SUPPORT)
-else()
+   else()
       remove_global_define(DLIB_JPEG_SUPPORT)
-endif()
+   endif()
 
 
-if (DLIB_USE_BLAS AND blas_found)
+   if (DLIB_USE_BLAS AND blas_found)
       add_global_define(DLIB_USE_BLAS)
-else()
+   else()
       remove_global_define(DLIB_USE_BLAS)
-endif()
+   endif()
 
-if (DLIB_USE_LAPACK AND lapack_found)
+   if (DLIB_USE_LAPACK AND lapack_found)
       add_global_define(DLIB_USE_LAPACK)
-else()
+   else()
       remove_global_define(DLIB_USE_LAPACK)
-endif()
+   endif()
 
 
-if (DLIB_ISO_CPP_ONLY)
+   if (DLIB_ISO_CPP_ONLY)
       add_global_define(DLIB_ISO_CPP_ONLY)
-else()
+   else()
       remove_global_define(DLIB_ISO_CPP_ONLY)
-endif()
+   endif()
 
 
-if (DLIB_NO_GUI_SUPPORT)
+   if (DLIB_NO_GUI_SUPPORT)
       add_global_define(DLIB_NO_GUI_SUPPORT)
-else()
+   else()
       remove_global_define(DLIB_NO_GUI_SUPPORT)
-endif()
+   endif()
 
-if (DLIB_ENABLE_STACK_TRACE)
+   if (DLIB_ENABLE_STACK_TRACE)
       add_global_define(DLIB_ENABLE_STACK_TRACE)
-else()
+   else()
       remove_global_define(DLIB_ENABLE_STACK_TRACE)
-endif()
+   endif()
 
 
-if (DLIB_ENABLE_ASSERTS)
+   if (DLIB_ENABLE_ASSERTS)
       add_global_define(ENABLE_ASSERTS)
-else()
+   else()
       remove_global_define(ENABLE_ASSERTS)
-endif()
+   endif()
+
 
+endif()