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