The NUMA code was enabled whenever numa.h and numaif.h are detected.
Unfortunately, the hosts' header files were being detected even then
cross compiling in the absence of a target libnuma.

Fix that by relying on the the presence of libnuma instead of the
presence of the header files. The test for libnuma does `AC_TRY_LINK`
which will fail if the test program (compiled for the target) can't
be linked against libnuma.

Test Plan:
Build on x86_64/linux and make sure NUMA works and cross compile to
armhf/linux.

Reviewers: austin, bgamari, hvr, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2329