It is useful to build with _XOPEN_SOURCE defined on AIX, enabling X/Open
and POSIX compatibility mode, to work around stray macros and other
bugs in the headers provided by the system and build compiler.
This patch adds the config to cmake to build with _XOPEN_SOURCE defined on AIX with a few exceptions. Google Test internals require access to platform specific thread info constructs on AIX so in that case we build with _ALL_SOURCE defined instead. Libclang also uses header which needs _ALL_SOURCE on AIX so we leave that as is as well..
We also add building on AIX with the large file API and doing CMake header checks with X/OPEN definitions so the results are consistent with the environment that will be present in the build.