With c26ce784 (Merge branch 'AndreyG/cmake/modernization', 2017-06-28),
we have recently introduced a regression in the way we are searching for
headers. We have made sure to always include our own headers first, but
due to the changes in c26ce784  this is no longer guaranteed. In fact,
this already leads the compiler into picking "config.h" from the
"deps/regex" dependency, if it is used.

Fix the issue by declaring our internal include directories up front,
before any of the other search directories is added.