build: avoid defining _GNU_SOURCE

_GNU_SOURCE defines the kitchen sink, and also prefers glibc definitions
where glibc and POSIX conflict. Even though POSIX is worth less than
toilet paper, we still prefer the POSIX definitions.

rar.c needs asprintf(), which is _GNU_SOURCE-only. So we define
_GNU_SOURCE too specifically for this file.

stream/rar.c

@@ -25,6 +25,9 @@
  * Preamble
  *****************************************************************************/
 
+// needed for asprintf()
+#define _GNU_SOURCE
+
 #include <assert.h>
 #include <limits.h>
 #include <stdio.h>