Export pkg-config dependencies via the CMake cache

This will be useful when building GLFW as a CMake object library and
linking it into a larger library that exports pkg-config information.

Partly based on #1307 by @a3f.

Closes #1307.

CMake/glfw3.pc.in

@@ -7,7 +7,7 @@ Name: GLFW
 Description: A multi-platform library for OpenGL, window and input
 Version: @GLFW_VERSION@
 URL: https://www.glfw.org/
-Requires.private: @GLFW_PKG_DEPS@
+Requires.private: @GLFW_PKG_CONFIG_REQUIRES_PRIVATE@
 Libs: -L${libdir} -l@GLFW_LIB_NAME@
-Libs.private: @GLFW_PKG_LIBS@
+Libs.private: @GLFW_PKG_CONFIG_LIBS_PRIVATE@
 Cflags: -I${includedir}