Disable execinfo.h and backtrace() for musl

--- src/backtrace.cc	2015-06-01 10:36:22.655599851 +0200
+++ src/backtrace.cc	2015-06-01 10:35:04.179600083 +0200
@@ -22,15 +22,17 @@
 #ifndef _WIN32
 #ifndef __APPLE__
 #include <cstdlib>
-
+#ifdef	__GLIBC__
 #include <execinfo.h>
 #endif
 #endif
+#endif
 
 std::string get_backtrace() {
 	std::string result("Backtrace:\n");
 #ifndef _WIN32
 #ifndef __APPLE__
+#ifdef	__GLIBC__
 #define BACKTRACE_STACKSIZE 24
 
 	void * stack[BACKTRACE_STACKSIZE];
@@ -43,5 +45,6 @@
 	free(list);
 #endif
 #endif
+#endif
 	return result;
 }
