cmake: Add compiler source tests.

Implement the 3 header checks currently run by configure:
- aom/aom_integer.h
- pthread.h
- unistd.h

Note: the aom_integer.h check is only present to maintain parity
with the behavior of the original build system. The output variable
it sets is not actually used in the aom sources.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: I53479a1253a977d6fdb7b0d6e5233686d79315e3
2 files changed