Replace ad-hoc CMake code with DEFINE_SYMBOL

This target property does what we were doing manually.

src/CMakeLists.txt

@@ -84,6 +84,7 @@ set_target_properties(glfw PROPERTIES
                       SOVERSION ${GLFW_VERSION_MAJOR}
                       POSITION_INDEPENDENT_CODE ON
                       C_STANDARD 99
+                      DEFINE_SYMBOL _GLFW_BUILD_DLL
                       FOLDER "GLFW3")
 
 target_compile_definitions(glfw PRIVATE _GLFW_USE_CONFIG_H)

src/glfw_config.h.in

@@ -45,8 +45,6 @@
 // Define this to 1 if building GLFW for OSMesa
 #cmakedefine _GLFW_OSMESA
 
-// Define this to 1 if building as a shared library / dynamic library / DLL
-#cmakedefine _GLFW_BUILD_DLL
 // Define this to 1 to use Vulkan loader linked statically into application
 #cmakedefine _GLFW_VULKAN_STATIC