Ask “brew” for the location instead. This is a follow-up
to commit 5931a94d

.

This will print “sh: brew: command not found” and
“Set the LLVM_INSTALL_DIR ...” if homebrew is not
installed.

Change-Id: I66c209a6db48b253bf55e24715ccf05f5b203cd9
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>