CMake: make compile definition "FL_DLL" public for MSVC (#960)

Visual Studio projects that link to the FLTK DLL (fltk::fltk-shared)
inherit the compile definition set by the DLL target and will be
compiled with "-D FL_DLL" as required w/o the user project having
to set this preprocessor macro explicitly.

Todo: documentation will follow...

CMake/fl_add_library.cmake

@@ -197,7 +197,7 @@ function(fl_add_library LIBNAME LIBTYPE SOURCES)
         OUTPUT_NAME_DEBUG   ${LIBNAME}_dlld
         OUTPUT_NAME_RELEASE ${LIBNAME}_dll
       )
-      target_compile_definitions(${TARGET_NAME} PRIVATE FL_DLL)
+      target_compile_definitions(${TARGET_NAME} PUBLIC FL_DLL)
     endif(MSVC)
   endif(LIBTYPE STREQUAL "SHARED")