cmake script to suppress randlib warnings about *.a having no symbols on MacOSX (#390)

* cmake script to suppress randlib warnings about *.a having no symbols on
MacOSX

* Moved script code into the main CMakeLists.txt file to suppress the
superfluous ranlib warnings all the time.

dlib/CMakeLists.txt

@@ -63,6 +63,14 @@ macro (toggle_preprocessor_switch option_name)
    endif()
 endmacro()
 
+# Suppress superfluous randlib warnings about libdlib.a having no symbols on MacOSX.
+if (APPLE)
+    set(CMAKE_C_ARCHIVE_CREATE   "<CMAKE_AR> Scr <TARGET> <LINK_FLAGS> <OBJECTS>")
+    set(CMAKE_CXX_ARCHIVE_CREATE "<CMAKE_AR> Scr <TARGET> <LINK_FLAGS> <OBJECTS>")
+    set(CMAKE_C_ARCHIVE_FINISH   "<CMAKE_RANLIB> -no_warning_for_no_symbols -c <TARGET>")
+    set(CMAKE_CXX_ARCHIVE_FINISH "<CMAKE_RANLIB> -no_warning_for_no_symbols -c <TARGET>")
+endif()
+
 # 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