[build] Fix some build issues caused by using tracy

Mostly just macro conflicts (and a little white space in passing).
Commits for integrating tracy will come later when I've come up with a
wrapper-api that I like (so non-tracy builds are easy even with tracy
available).

libs/gib/regex.c

@@ -24,7 +24,9 @@
   #pragma alloca
 #endif
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 
 /* We need this for `regex.h', and perhaps for the Emacs include files.  */
 #include <sys/types.h>
@@ -241,8 +243,12 @@ char *alloca ();
 
 #define STREQ(s1, s2) ((strcmp (s1, s2) == 0))
 
+#ifndef MAX
 #define MAX(a, b) ((a) > (b) ? (a) : (b))
+#endif
+#ifndef MIN
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
+#endif
 
 typedef char boolean;
 #define false 0

libs/ruamoko/rua_math.c

@@ -31,7 +31,9 @@
 # include "config.h"
 #endif
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 #include <stdlib.h>
 #include <math.h>
 

libs/ruamoko/rua_stdlib.c

@@ -31,7 +31,9 @@
 # include "config.h"
 #endif
 
-#define _GNU_SOURCE	// for qsort_r
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
 
 #ifdef HAVE_STRING_H
 # include <string.h>