build: Remove execinfo.h check

The check for the execinfo.h header is only advisory; the build will not
fail if it is not present, and set HAVE_EXECINFO_H if it is. The check
was added in commit bc3e020475e ("build: Add declaration checks to check for
required syscall flags") with no obvious use or reasoning.

Remove the no-op check.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/configure.ac b/configure.ac
index 81cf407..8c2fb82 100644
--- a/configure.ac
+++ b/configure.ac
@@ -109,7 +109,6 @@
 	AC_CHECK_DECL(CLOCK_MONOTONIC,[],
 		      [AC_MSG_ERROR("CLOCK_MONOTONIC is needed to compile wayland libraries")],
 		      [[#include <time.h>]])
-	AC_CHECK_HEADERS([execinfo.h])
 fi
 
 PKG_CHECK_MODULES(EXPAT, [expat], [],