configure: remove use of AS_VAR_APPEND

This wasn't used often and benefits were likely minimal. Dropping it
outright is a bit simpler than adding a compatibility ifdef.

provides some compatibility with older versions of autoconf.
tested with autoconf 2.59/automake 1.7/aclocal 1.7

Change-Id: Ifed892346cf2329597985704830a96fc58d65607
(cherry picked from commit 9326a56f8d)

configure.ac

@@ -40,7 +40,10 @@ AC_DEFUN([TEST_AND_ADD_CFLAGS],
           dnl Note AC_LANG_PROGRAM([]) uses an old-style main definition.
           AC_COMPILE_IFELSE([AC_LANG_SOURCE([int main(void) { return 0; }])],
                             [AC_MSG_RESULT([yes])]
-                            [AS_VAR_APPEND([AM_CFLAGS], [" $1"])],
+                            dnl Simply append the variable avoiding a
+                            dnl compatibility ifdef for AS_VAR_APPEND as this
+                            dnl variable shouldn't grow all that large.
+                            [AM_CFLAGS="$AM_CFLAGS $1"],
                             [AC_MSG_RESULT([no])])
           CFLAGS="$SAVED_CFLAGS"])
 TEST_AND_ADD_CFLAGS([-Wall])