Installation directories

[I am not on this list, so please cc me.]

It seems that clang from SVN (compiled on OS X 10.7) installs in /usr/local/ but expects libraries and header to be in /usr/lib/ and /usr/include/. At least the library libc++.1.dylib will then clash with the one installed by Xcode. So it might be better to put all stuff in /usr/local/ for a user install (as GCC 7.1 does).