graphics/libfpx fails to build

[Kenta Suzumoto]

Hi all. graphics/libfpx seems to fail for me in poudriere.

Here's the whole log http://dpaste.com/1363160/plain/

Building with GCC in base instead of Clang seems to fix it. Maybe we can update the Makefile?

There's also a PR to update it which may fix it, but the maintainer hasn't responded in a few weeks.

https://www.freebsd.org/cgi/query-pr.cgi?pr=181302