Remove makefile cruft.

The files that need _GNU_SOURCE define it themselves, and bionic
now correctly takes that to imply the latest _POSIX_C_SOURCE (which
this _wasn't_ getting, leading to a missing declaration for open_memstream).

Bug: 17361504
Change-Id: Idd1c2abb6818d5216a7cdcdbaa235a113447831f
1 file changed