compat/regex: get the gawk regex engine to compile within git

We need to define -DGAWK -DNO_MBSUPPORT so that the gawk regex engine
will compile, and include stdio.h and stddef.h in regex.h. Gawk itself
includes these headers before it includes the regex.h header.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>