Squash glibc warning about _BSD_SOURCE being deprecated tests.

Squash glibc warning about _BSD_SOURCE being deprecated tests.

glibc 2.20 deprecated _BSD_SOURCE and _SVID_SOURCE and now requires
_DEFAULT_SOURCE to be defined instead. Add _DEFAULT_SOURCE define to
prevent warnings like:

In file included from /usr/include/stdio.h:27:0,
                 from stack_switch.c:5:
/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"
   ^

See https://sourceware.org/glibc/wiki/Release/2.20#Packaging_Changes

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15768