docs: Strongly recommend setting rpath when using a local GCC toolchain

Users keep emailing us about the difficulties of getting LD_LIBRARY_PATH
into their environment, which should be completely unecessary. Try to
strengthen the rpath recommentation by putting in an example cmake
invocation.

Speaking of which, we might want to make CMake the recommended build
system in GettingStarted.html.

llvm-svn: 214565