Convert additional cflags comments into accessible pkg-config's vars.

The additional flags are usually trivial on *nix platforms but they are not so on Windows platform for MSVC.

Source/Engine/Urho3D.pc.in

@@ -25,13 +25,14 @@ exec_prefix=${prefix}
 libdir=${exec_prefix}/@DEST_LIBRARY_DIR@
 includedir=${prefix}/@DEST_INCLUDE_DIR@
 
+# Additional Cflags for various build configurations, which can be accessed as normal pkg-config variable using '--variable' option
+CFLAGS_DEBUG=@CMAKE_CXX_FLAGS_DEBUG@
+CFLAGS_RELEASE=@CMAKE_CXX_FLAGS_RELEASE@
+CFLAGS_RELWITHDEBINFO=@CMAKE_CXX_FLAGS_RELWITHDEBINFO@
+
 Name: Urho3D
 Description: Urho3D is a lightweight, cross-platform rendering and game engine implemented in C++ and released under the MIT license. Greatly inspired by OGRE (http://www.ogre3d.org) and Horde3D (http://www.horde3d.org).
 Version: 1.3.0
 URL: https://code.google.com/p/urho3d/
 Libs: @CMAKE_EXE_LINKER_FLAGS@ @URHO3D_ABS_PATH_LIBS@ -L${libdir} @URHO3D_LIBS@
 Cflags: @URHO3D_COMPILE_DEFINITIONS@ @CMAKE_CXX_FLAGS@ @GLOBAL_INCLUDE_DIRS@ @URHO3D_INCLUDE_DIRS@
-# Additional Cflags for:
-# DEBUG: @CMAKE_CXX_FLAGS_DEBUG@
-# RELEASE: @CMAKE_CXX_FLAGS_RELEASE@
-# RELWITHDEBINFO: @CMAKE_CXX_FLAGS_RELWITHDEBINFO@