cmake: define WIN_RC with other platform sources

This makes splitting up the library build instructions later on more
obvious and easier to achieve.

CMakeLists.txt

@@ -265,8 +265,6 @@ IF (MSVC)
 	SET(CMAKE_SHARED_LINKER_FLAGS_RELEASE "${CMAKE_EXE_LINKER_FLAGS_RELEASE}")
 	SET(CMAKE_SHARED_LINKER_FLAGS_RELWITHDEBINFO "${CMAKE_EXE_LINKER_FLAGS_RELWITHDEBINFO}")
 	SET(CMAKE_SHARED_LINKER_FLAGS_MINSIZEREL "${CMAKE_EXE_LINKER_FLAGS_MINSIZEREL}")
-
-	SET(WIN_RC "src/win32/git2.rc")
 ELSE ()
 	SET(CMAKE_C_FLAGS "-D_GNU_SOURCE ${CMAKE_C_FLAGS}")
 
@@ -593,6 +591,11 @@ FILE(GLOB SRC_H include/git2.h include/git2/*.h include/git2/sys/*.h)
 # On Windows use specific platform sources
 IF (WIN32 AND NOT CYGWIN)
 	ADD_DEFINITIONS(-DWIN32 -D_WIN32_WINNT=0x0501)
+
+	IF(MSVC)
+		SET(WIN_RC "src/win32/git2.rc")
+	ENDIF()
+
 	FILE(GLOB SRC_OS src/win32/*.c src/win32/*.h)
 ELSEIF (AMIGA)
 	ADD_DEFINITIONS(-DNO_ADDRINFO -DNO_READDIR_R -DNO_MMAP)