Fix cross-platform building

Commit 45c7237 ("Fixup for recent firmware inclusion changes")
breaks builds on non-Apple platforms. Fix it for platforms that
already have fmemopen - others may still need attention.

See: raspberrypi#47

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

main.c

@@ -9,7 +9,7 @@
 #include "msd/start.h"
 
 /* Assume BSD without native fmemopen() if doesn't seem to be glibc */
-#if defined(__APPLE__) || !defined(_GNU_SOURCE) || !defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200809L
+#if defined(__APPLE__) || (!defined(_GNU_SOURCE) && (!defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200809L))
 #include "fmemopen.c" // BSD fmemopen() compat in terms of funopen()
 #endif