Manually build cquery doesn't find system headers on macOS

[nsabovic]

I tried to build it manually, using the instructions. I'm running macOS 10.14.2 with Xcode 10.1 I checked out master (70c755b) and build with:

-DCMAKE_BUILD_TYPE=Release -DSYSTEM_CLANG=OFF -DCMAKE_PREFIX_PATH=/opt/cquery

It can't find most of the system headers. Running /opt/cquery/bin/cquery --check on a file gives errors such as:

(   0.952s) [       36F9BB]  clang_indexer.cc:2366  | /usr/local/Cellar/glib/2.58.2/include/glib-2.0/glib/gtypes.h:35:10 'time.h' file not found

Running the Homebrew version /usr/local/Cellar/cquery/HEAD-70c755b/bin/cquery --check on the same file works just fine.