Define explicit _WIN32_WINNT version in makefile

Previously, it was defined in netops.c, but it's also needed in one of the
clar tests, so I figured we might as well just make it global for the
whole project.

Without it, the mingw32 linker won't resolve GetProcessId() (called from
the core/errors.c clar test) because of some conditionals in windows.h.

CMakeLists.txt

@@ -96,7 +96,7 @@ FILE(GLOB SRC_H include/git2/*.h)
 
 # On Windows use specific platform sources
 IF (WIN32 AND NOT CYGWIN)
-	ADD_DEFINITIONS(-DWIN32 -D_DEBUG)
+	ADD_DEFINITIONS(-DWIN32 -D_DEBUG -D_WIN32_WINNT=0x0501)
 	FILE(GLOB SRC src/*.c src/transports/*.c src/xdiff/*.c src/win32/*.c)
 ELSE()
 	FILE(GLOB SRC src/*.c src/transports/*.c src/xdiff/*.c src/unix/*.c)

src/netops.c

@@ -11,7 +11,6 @@
 #	include <sys/time.h>
 #	include <netdb.h>
 #else
-#	define _WIN32_WINNT 0x0501
 #	include <winsock2.h>
 #	include <Ws2tcpip.h>
 #	ifdef _MSC_VER