Drop support for THREADSAFE on Windows XP

This makes libgit2 require Windows Vista or newer if it is going
to be compiled with the THREADSAFE option

CMakeLists.txt

@@ -287,8 +287,13 @@ 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)
+	ADD_DEFINITIONS(-DWIN32)
 	FILE(GLOB SRC_OS src/win32/*.c src/win32/*.h)
+	IF (THREADSAFE)
+		ADD_DEFINITIONS(-D_WIN32_WINNT=0x0600)
+	ELSE()
+		ADD_DEFINITIONS(-D_WIN32_WINNT=0x0501)
+	ENDIF()
 ELSEIF (AMIGA)
 	ADD_DEFINITIONS(-DNO_ADDRINFO -DNO_READDIR_R)
 	FILE(GLOB SRC_OS src/amiga/*.c src/amiga/*.h)

src/thread-utils.h

@@ -40,6 +40,10 @@ typedef git_atomic git_atomic_ssize;
 
 #ifdef GIT_THREADS
 
+#if defined(GIT_WIN32) && _WIN32_WINNT < 0x0600
+#	error "Unsupported Windows version for thread support"
+#endif
+
 #define git_thread pthread_t
 #define git_thread_create(thread, attr, start_routine, arg) \
 	pthread_create(thread, attr, start_routine, arg)
@@ -62,7 +66,15 @@ typedef git_atomic git_atomic_ssize;
 #define git_cond_signal(c)	pthread_cond_signal(c)
 #define git_cond_broadcast(c)	pthread_cond_broadcast(c)
 
-/* Pthreads rwlock */
+/* Pthread (-ish) rwlock
+ *
+ * This differs from normal pthreads rwlocks in two ways:
+ * 1. Separate APIs for releasing read locks and write locks (as
+ *    opposed to the pure POSIX API which only has one unlock fn)
+ * 2. You should not use recursive read locks (i.e. grabbing a read
+ *    lock in a thread that already holds a read lock) because the
+ *    Windows implementation doesn't support it
+ */
 #define git_rwlock pthread_rwlock_t
 #define git_rwlock_init(a)		pthread_rwlock_init(a, NULL)
 #define git_rwlock_rdlock(a)	pthread_rwlock_rdlock(a)