main: Fix __iwd_backtrace_init() availability detection

Check for HAVE_EXECINFO_H for all __iwd_backtrace_init usages.

Fixes:

  src/main.o: In function `main':
  main.c:(.text.startup+0x798): undefined reference to `__iwd_backtrace_init'
  collect2: error: ld returned 1 exit status

src/backtrace.h

@@ -19,7 +19,7 @@
  *  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  *
  */
-#ifdef __GLIBC__
+#ifdef HAVE_EXECINFO_H
 void __iwd_backtrace_init();
 void __iwd_backtrace_print(unsigned int offset);
 #endif

src/main.c

@@ -445,7 +445,7 @@ int main(int argc, char *argv[])
 	if (debugopt)
 		l_debug_enable(debugopt);
 
-#ifdef __GLIBC__
+#ifdef HAVE_EXECINFO_H
 	__iwd_backtrace_init();
 #endif