Quote AC_SUBST within m4_ifval, so it does not get prematurely expanded.

(closes issue #17654)
 Reported by: pprindeville
 Patches: 
       issue17654.diff uploaded by qwell (license 4)
 Tested by: qwell, pprindeville


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@276830 65c4cc65-6c06-0410-ace0-fbb531ad65f3

autoconf/ast_gcc_attribute.m4

@@ -29,7 +29,7 @@ AC_COMPILE_IFELSE(
 )
 fi
 
-m4_ifval([$4],AC_SUBST($4))
+m4_ifval([$4],[AC_SUBST($4)])
 CFLAGS="$saved_CFLAGS"
 ]
 )

configure

 #! /bin/sh
-# From configure.ac Revision: 276731 .
+# From configure.ac Revision: 276769 .
 # Guess values for system-dependent variables and create Makefiles.
 # Generated by GNU Autoconf 2.61 for asterisk trunk.
 #
@@ -1087,7 +1087,6 @@ POW_LIB
 HAS_POLL
 PBX_PTHREAD_RWLOCK_INITIALIZER
 PBX_PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP
-
 PBX_WEAKREF
 GC_CFLAGS
 GC_LDFLAGS