AIX: Stop adding option -qsrcmsg.

With xlc v16.1.0, it causes internal compiler errors.  With xlc versions
not exhibiting that bug, removing -qsrcmsg merely changes the compiler
error reporting format.  Back-patch to 9.4 (all supported versions).

Discussion: https://postgr.es/m/20191003064105.GA3955242@rfd.leadboat.com

src/template/aix

-# Set appropriate default compiler options if using xlc
-# Note: configure will add -qnoansialias if the compiler accepts it,
-# even if user specifies a non-default CFLAGS setting.
+# Set default options if using xlc.  This formerly included -qsrcmsg, but that
+# option elicits internal compiler errors from xlc v16.1.0.  Note: configure
+# will add -qnoansialias if the compiler accepts it, even if user specifies a
+# non-default CFLAGS setting.
 if test "$GCC" != yes ; then
   case $host_os in
     aix3.2.5 | aix4.1*)
-      CFLAGS="-O -qmaxmem=16384 -qsrcmsg"
+      CFLAGS="-O -qmaxmem=16384"
       ;;
     *)
-      CFLAGS="-O2 -qmaxmem=16384 -qsrcmsg"
+      CFLAGS="-O2 -qmaxmem=16384"
       ;;
   esac