Add option to build API exporting object library

This will cause the GLFW public API to be exported by any shared
library or executable that links the GLFW object library.

This option is only available when GLFW_LIBRARY_TYPE is set to OBJECT.

README.md

@@ -131,6 +131,8 @@ information on what to include when reporting a bug.
    surface extension (#1793)
  - Added `GLFW_LIBRARY_TYPE` CMake variable for overriding the library type
    (#279,#1307,#1497,#1574,#1928)
+ - Added `GLFW_EXPORT_PUBLIC_API` CMake option for building an object library that exports
+   the public API
  - Made joystick subsystem initialize at first use (#1284,#1646)
  - Made `GLFW_DOUBLEBUFFER` a read-only window attribute
  - Updated the minimum required CMake version to 3.1

docs/compile.dox

@@ -271,6 +271,12 @@ with the library.  This is enabled by default if
 __GLFW_VULKAN_STATIC__ determines whether to use the Vulkan loader linked
 directly with the application.  This is disabled by default.
 
+@anchor GLFW_EXPORT_PUBLIC_API
+__GLFW_EXPORT_PUBLIC_API__ is only available when building GLFW as a CMake object library.
+It determines whether the object files export the public GLFW API.  This is useful for
+example when linking GLFW into in a larger shared library.  See @ref GLFW_LIBRARY_TYPE for
+how to build an object library.
+
 
 @subsection compile_options_win32 Windows specific CMake options
 

src/CMakeLists.txt

@@ -100,9 +100,17 @@ set_target_properties(glfw PROPERTIES
                       POSITION_INDEPENDENT_CODE ON
                       C_STANDARD 99
                       C_EXTENSIONS OFF
-                      DEFINE_SYMBOL _GLFW_BUILD_DLL
                       FOLDER "GLFW3")
 
+if (GLFW_BUILD_SHARED_LIBRARY OR GLFW_EXPORT_PUBLIC_API)
+    target_compile_definitions(glfw PRIVATE _GLFW_BUILD_DLL)
+endif()
+
+if (UNIX)
+    # Hide symbols not explicitly tagged for export from the shared library
+    target_compile_options(glfw PRIVATE "-fvisibility=hidden")
+endif()
+
 target_include_directories(glfw PUBLIC
                            "$<BUILD_INTERFACE:${GLFW_SOURCE_DIR}/include>"
                            "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>")
@@ -230,11 +238,6 @@ if (GLFW_BUILD_SHARED_LIBRARY)
         # Clear flags again to avoid breaking later tests
         set(CMAKE_REQUIRED_FLAGS)
     endif()
-
-    if (UNIX)
-        # Hide symbols not explicitly tagged for export from the shared library
-        target_compile_options(glfw PRIVATE "-fvisibility=hidden")
-    endif()
 endif()
 
 if (GLFW_INSTALL)