Make CMake build dlib statically whenever DLIB_IN_PROJECT_BUILD==true,

regardless of the state of BUILD_SHARED_LIBS.  This means projects that
use dlib by saying add_subdirectory(dlib) will always statically link to
dlib, unless DLIB_IN_PROJECT_BUILD is explicltly set to false and
BUILD_SHARED_LIBS set to true.

CMakeLists.txt

@@ -2,14 +2,35 @@ cmake_minimum_required(VERSION 2.8.12)
 
 
 
+
+#############################################################################
+#                                                                           #
+#  READ examples/CMakeLists.txt TO SEE HOW TO USE DLIB FROM C++ WITH CMAKE  #
+#                                                                           #
+#############################################################################
+
+
+
+
+
 get_directory_property(has_parent PARENT_DIRECTORY)
 if(NOT has_parent)
-   # Set this so that the dlib subfolder will build both a static and shared
-   # library, since the only reason to build this CMakeLists.txt by itself is
-   # if you want to install dlib.  It should be noted however that installing
-   # dlib is not necessary.  A simpler approach is to use it the way shown in
-   # examples/CMakeLists.txt 
+   # When you call add_subdirectory(dlib) from a parent CMake project dlib's
+   # CMake scripts will assume you want to statically compile dlib into
+   # whatever you are building rather than create a standalone copy of dlib.
+   # This means CMake will build dlib as a static library, disable dlib's
+   # install targets so they don't clutter your project, and adjust a few other
+   # minor things that are convenient when statically building dlib as part of
+   # your own projects.
+   #
+   # On the other hand, if there is no parent CMake project or if
+   # DLIB_IN_PROJECT_BUILD is set to false, CMake will compile dlib as a normal
+   # standalone library (either shared or static, based on the state of CMake's
+   # BUILD_SHARED_LIBS flag), and include the usual install targets so you can
+   # install dlib on your computer via `make install`.  Since the only reason
+   # to build this CMakeLists.txt (the one you are reading right now) by itself
+   # is if you want to install dlib, we indicate as such by setting
+   # DLIB_IN_PROJECT_BUILD to false.
    set(DLIB_IN_PROJECT_BUILD false)
 endif()
-
 add_subdirectory(dlib)

dlib/CMakeLists.txt

@@ -32,6 +32,31 @@ if(has_parent)
    endif()
 endif()
 
+
+if (DLIB_IN_PROJECT_BUILD)
+   # DLIB_IN_PROJECT_BUILD==true means you are using dlib by invoking
+   # add_subdirectory(dlib) in the parent project. In this case, we always want
+   # to build dlib as a static library so the parent project doesn't need to
+   # deal with some random dlib shared library file.  It is much better to
+   # statically compile dlib into the parent project.  So the following bit of
+   # CMake ensures that happens.  However, we have to take care to compile dlib
+   # with position independent code if appropriate (i.e. if the parent project
+   # is a shared library).
+   if (BUILD_SHARED_LIBS)
+      if (CMAKE_COMPILER_IS_GNUCXX)
+         # Just setting CMAKE_POSITION_INDEPENDENT_CODE should be enough to set
+         # -fPIC for GCC but sometimes it still doesn't get set, so make sure it
+         # does.
+         add_definitions("-fPIC")
+      endif()
+      set(CMAKE_POSITION_INDEPENDENT_CODE true)
+   endif()
+
+   # Tell cmake to build dlib as a static library
+   set(BUILD_SHARED_LIBS false)
+endif()
+
+
 if (CMAKE_VERSION VERSION_LESS "3.9.0")
    # Set only because there are old target_link_libraries() statements in the
    # FindCUDA.cmake file that comes with CMake that error out if the new behavior
@@ -710,6 +735,9 @@ if (NOT TARGET dlib)
    if (DLIB_IN_PROJECT_BUILD)
       target_compile_options(dlib PUBLIC ${active_preprocessor_switches})
    else()
+      # These are private in this case because they will be controlled by the
+      # contents of dlib/config.h once it's installed. But for in project
+      # builds, there is no real config.h so they are public in the above case.
       target_compile_options(dlib PRIVATE ${active_preprocessor_switches})
    endif()