cmake: BSD: Mark /usr/local/include as system include directory

We add /usr/local/include to the include directory list for some BSD
systems. We should mark this as a system directory to avoid files from
/usr/local/include getting picked over files shipping with llvm.

This typically manifested as gtest headers installed with the system
getting picked over the ones shipping with llvm.

Patch by Petr Penzin <penzin.dev@gmail.com>

Differential Revision: https://reviews.llvm.org/D37415

llvm-svn: 315952