There is no <execinfo.h> in musl libc

--- libvtv/vtv_rts.cc	2016-09-23 16:17:11.000000000 +0200
+++ libvtc/vtv_rts.cc	2016-09-25 15:08:57.896000000 +0200
@@ -124,7 +124,7 @@
 #include <windows.h>
 #include <winternl.h>
 #include <psapi.h>
-#else
+#elif defined (__GLIBC__)
 #include <execinfo.h>
 #endif
 
--- libvtv/vtv_utils.cc	2016-09-23 16:17:11.000000000 +0200
+++ libvtv/vtv_utils.cc	2016-09-25 15:15:35.529000000 +0200
@@ -34,7 +34,7 @@
 #include <string.h>
 #if defined (__CYGWIN__) || defined (__MINGW32__)
 #include <windows.h>
-#else
+#elif defined (__GLIBC__)
 #include <execinfo.h>
 #endif
 
@@ -178,7 +178,7 @@
 
   __vtv_add_to_log (vtv_failures_log_fd, "%s", log_msg);
 
-#if !defined (__CYGWIN__) && !defined (__MINGW32__)
+#if !defined (__CYGWIN__) && !defined (__MINGW32__) && defined(__GLIBC__)
   if (generate_backtrace)
     {
 #define STACK_DEPTH 20