Teach clang to look for libcxx in /usr/local/include/c++ on Linux
Teach clang to look for libcxx in /usr/local/include/c++ on Linux

As The default CMAKE install prefix is /usr/local ( https://cmake.org/cmake/help/v3.0/variable/CMAKE_INSTALL_PREFIX.html ),
sudo ninja install ends up installing clang, LLVM and libcxx under /usr/local.
In development scenario, when clang is run from the build location it will not
find libcxx at neither (build location)/../include/c++ nor /usr/include/c++.
This patch lets development clang find system installed libcxx without adding
-isystem /usr/local/include/c++. Also addresses the FIXME by explaining the 
use-case for these include paths.



git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@269801 91177308-0d34-0410-b5e6-96231b3b80d8

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