Unbreak build with devel/utf8cpp installed

Let preprocessor know ${LOCALBASE}/include is a system include path,
so #include "..." shouldn't try to pick headers from there. lang/gcc*
and lang/clang* already do the right thing.

PR: 196220
Differential Revision: https://reviews.freebsd.org/D1664
Approved by: garga (maintainer)
Approved by: bapt (mentor)