Use "grep -E" not "egrep".

Use "grep -E" not "egrep".

"egrep" has never been in POSIX; the standard way to access this
functionality is "grep -E".  Recent versions of GNU grep have
started to warn about this, so stop using "egrep".

This could be back-patched, but I see little need to do so
because the affected places are not code that runs during
normal builds.  (Perhaps src/backend/port/aix/mkldexport.sh
is an exception, but let's wait to see if any AIX users
complain before touching that.)

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/473272.1778685870@sss.pgh.pa.us