Correctly set resource dir when not specified explicitly

[Xazax-hun]

Right now we detect default include directories of gcc. This might interfere with clang resource dirs. We need to include the clang resource dir with higher priority than the detected gcc includes (but potentially lower than user include dirs?). To address the priority issue we specify the clang resource dir with the -isystem flag. This, however, is not specified when the resource dir is not set in the package layout. From clang 5.0 it is easy to query the resource dir from clang. We should query the resource dir from clang in case it is not specified in the package layout config.