Disable <execinfo.h> and backtrace(3) if not GNU libc

--- src/stacktrace.hpp	2015-11-03 09:17:30.000000000 +0100
+++ src/stacktrace.hpp	2015-11-06 11:03:32.001951469 +0100
@@ -5,7 +5,9 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#if defined(__GLIBC__)
 #include <execinfo.h>
+#endif
 #include <cxxabi.h>
 
 #include "SDL.h"
@@ -13,6 +15,7 @@
 /** Print a demangled stack backtrace of the caller function to FILE* out. */
 static inline void print_stacktrace(unsigned int max_frames = 63)
 {
+#if defined(__GLIBC__)
 	SDL_LogCritical(SDL_LOG_CATEGORY_APPLICATION, "%s\n", "stack trace:");
 
     // storage array for stack trace address data
@@ -79,4 +82,7 @@
 
 	free(funcname);
     free(symbollist);
+#else
+	SDL_LogCritical(SDL_LOG_CATEGORY_APPLICATION, "%s\n", "stack trace is not available for your architecture");
+#endif
 }