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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@315952 91177308-0d34-0410-b5e6-96231b3b80d8

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