includes/Stg.h: define _DEFAULT_SOURCE for glibc-2.20

_BSD_SOURCE we are using for 'gamma()' and friends
was deprecated in glibc-2.20 in favour of '_DEFAULT_SOURCE'.

gcc says:

    In file included from /usr/include/math.h:26:0:
        0,
                         from includes/Stg.h:69,
                         from /tmp/ghc19488_0/ghc19488_2.hc:3:

    /usr/include/features.h:148:3:
         warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Wcpp]
         # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
           ^

Patch fixes testsuite failures on UNREG
(stderr are not cluttered by warnings anymore).

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>