compat/regex: use the regex engine from gawk for compat

Change the regex engine in compat to use the gawk engine from the
gawk-devel module in gawk CVS. This engine supports the REG_STARTEND
flag, which was optionally available in Git since v1.7.2-rc0~77^2~1.

The source was grabbed from cvs.savannah.gnu.org:/sources/gawk, and
these are the upstream versions of the files being included:

    regcomp.c               1.4
    regex.h                 1.3
    regex.h                 1.3
    regex_internal.c        1.3
    regex_internal.h        1.3
    regexec.c               1.3
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>