postgresql: enable uclibc build

postgresql: enable uclibc build

Build with uclibc and locale support failes with the following error
message:

regc_pg_locale.c: In function ‘pg_wc_isdigit’:
regc_pg_locale.c:312:6: error: dereferencing pointer to incomplete type
      isdigit_l((unsigned char) c, pg_regex_locale));

This can be fixed by overwriting the HAVE_LOCALE_T detection (thanks
to Alex Potapenko <opotapenko@gmail.com> for the hint [1]).

[1] http://lists.busybox.net/pipermail/buildroot/2015-March/121088.html

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>