CMake: Only add the STDCALL option when using MSVC

This option is only ever used with MSVC.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>

CMakeLists.txt

@@ -47,10 +47,14 @@ SET(INSTALL_INC include CACHE PATH "Where to install headers to.")
 # Build options
 OPTION (BUILD_SHARED_LIBS "Build Shared Library (OFF for Static)" ON)
 OPTION (THREADSAFE "Build libgit2 as threadsafe" OFF)
-OPTION (STDCALL "Buildl libgit2 with the __stdcall convention (Windows)" ON)
 OPTION (BUILD_TESTS "Build Tests" ON)
 OPTION (BUILD_CLAY "Build Tests using the Clay suite" OFF)
 
+# Not using __stdcall with the CRT causes problems
+IF (MSVC)
+	OPTION (STDCALL "Buildl libgit2 with the __stdcall convention" ON)
+ENDIF ()
+
 # Platform specific compilation flags
 IF (MSVC)
 	SET(CMAKE_C_FLAGS "/W4 /nologo /Zi")