testsuite, c-c++-common, Darwin: Define _DARWIN_C_SOURCE for quick_*.

glibc and Darwwin's libc have different (but mutually conforming) meanings
for _POSIX_SOURCE.  In order to gain the functions specified outside of the
posix case on darwin, we need to also spcify _DARWIN_C_SOURCE or the set
will be constrined to only those specified by the Posix level.

gcc/testsuite/ChangeLog:

	* c-c++-common/analyzer/flex-with-call-summaries.c: Specify
	_DARWIN_C_SOURCE.
	* c-c++-common/analyzer/flex-without-call-summaries.c: Likewise.
	* c-c++-common/analyzer/strndup-1.c: Likewise.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>