util: build with external libatomic

util: build with external libatomic

Recent versions of gcc on some architectures (MIPS, PPC, ...) moved
atomic functions into a separate library. As we are using atomic
load/store in util/time.c we may need to link libgnunetutil against
libatomic for __atomic_load_8 and __atomic_store_8 to be defined.

Fixes build problem on MIPS:
ld: ./.libs/libgnunetutil.so: undefined reference to `__atomic_store_8'
ld: ./.libs/libgnunetutil.so: undefined reference to `__atomic_load_8'
collect2: error: ld returned 1 exit status

Signed-off-by: Daniel Golle <daniel@makrotopia.org>