Use __GNUC__ macro in the resource script

Fix the default LIBGIT2_FILENAME for GNU windres

src/CMakeLists.txt

@@ -278,10 +278,6 @@ list(SORT SRC_H)
 
 # On Windows use specific platform sources
 if(WIN32 AND NOT CYGWIN)
-	if(NOT MSVC)
-		ADD_DEFINITIONS(-DGCC_WINDRES)
-	endif()
-
 	SET(WIN_RC "win32/git2.rc")
 
 	file(GLOB SRC_OS win32/*.c win32/*.h)

src/win32/git2.rc

@@ -2,21 +2,25 @@
 #include "../../include/git2/version.h"
 
 #ifndef LIBGIT2_FILENAME
-# define LIBGIT2_FILENAME "git2"
+# ifdef __GNUC__
+#  define LIBGIT2_FILENAME git2
+# else
+#  define LIBGIT2_FILENAME "git2"
+# endif
 #endif
 
 #ifndef LIBGIT2_COMMENTS
 # define LIBGIT2_COMMENTS "For more information visit http://libgit2.github.com/"
 #endif
 
-#ifdef GCC_WINDRES
+#ifdef __GNUC__
 # define _STR(x) #x
 # define STR(x) _STR(x)
 #else
 # define STR(x) x
 #endif
 
-#ifdef GCC_WINDRES
+#ifdef __GNUC__
 VS_VERSION_INFO		VERSIONINFO
 #else
 VS_VERSION_INFO		VERSIONINFO	MOVEABLE IMPURE LOADONCALL DISCARDABLE