Main: fix compilation on OS X using non-Apple compilers

Commit d42c0f1dc5 added an OS X-specific
method of calling mpd_main_after_fork(), which uses Grand Central
Dispatch. Since this uses a block literal, it breaks compilation on
compilers which don't support the block extension, e.g. non-Apple
compilers. This affects users on older OS X releases with GCD (which
depend on older Clang releases, or Apple GCCs, which don't support the
C++11 features MPD needs); or which don't support GCD at all (10.5 and
lower).

This patch changes the #ifdef so that the non-GCD code is used
as it was on OS X before this patch if blocks aren't available, via
checking __BLOCKS__ macro.

src/Main.cxx

@@ -114,7 +114,7 @@
 #include <ws2tcpip.h>
 #endif
 
-#ifdef __APPLE__
+#ifdef __BLOCKS__
 #include <dispatch/dispatch.h>
 #endif
 
@@ -517,7 +517,7 @@ int mpd_main(int argc, char *argv[])
 	daemonize_begin(options.daemon);
 #endif
 
-#ifdef __APPLE__
+#ifdef __BLOCKS__
 	/* Runs the OS X native event loop in the main thread, and runs
 	   the rest of mpd_main on a new thread. This lets CoreAudio receive
 	   route change notifications (e.g. plugging or unplugging headphones).