Removing -qsrcmsg (AIX)

With AIX xlc v16.1.0, various compilations get internal errors:

/opt/IBM/xlc/16.1.0/bin/xlc_r -qmaxmem=33554432 -qnoansialias -g -O2 -qsrcmsg -I/scratch/nm/farmlike/../cmpx/src/interfaces/libpq -I../../../src/include -I/scratch/nm/farmlike/../cmpx/src/include    -c -o pg_backup_utils.o /scratch/nm/farmlike/../cmpx/src/bin/pg_dump/pg_backup_utils.c
/opt/IBM/xlc/16.1.0/bin/.orig/xlc_r: 1501-230 (S) Internal compiler error; please contact your Service Representative. For more information visit:
http://www.ibm.com/support/docview.wss?uid=swg21110810

I tracked this down to our use of -qsrcmsg, which I plan to remove from the
default CFLAGS, including back branches.  While this is a compiler bug, it's
not worth breaking plausible builds to default-enable the minor change
-qsrcmsg provides:

$ echo 'break me' >x.c
$ /opt/IBM/xlc/16.1.0/bin/xlc_r x.c
"x.c", line 1.1: 1506-275 (S) Unexpected text 'break' encountered.
"x.c", line 1.7: 1506-166 (S) Definition of function me requires parentheses.
"x.c", line 1.9: 1506-204 (S) Unexpected end of file.
$ /opt/IBM/xlc/16.1.0/bin/xlc_r -qsrcmsg x.c
        1 | break me
            a.....b.c
a - 1506-275 (S) Unexpected text 'break' encountered.
b - 1506-166 (S) Definition of function me requires parentheses.
c - 1506-204 (S) Unexpected end of file.