textproc/mgdiff: Fix segfault when with open file dialogue box

Under open-motif we see a segfault when mgdiff calls regexec() in
libgnuregexec's regexec.c at line 782. Let's avoid libgnuregexec
entirely and use the regexec(3) provided by FreeBSD's libc instead.
This resolves the segfault and as a bonus we reduce mgdiff's
dependency list.

We also remove a now incorrect comment in the patch. It was misleading
since the EXTRA_LIBRARIES for Dell SVR4 had already been removed.

PR: 280289
Approved by: portmaster@BSDforge.com (maintainer)
MFH: 2024Q3