Starting with Xcode 4.3, you cannot just install Xcode and expect usual Unux application command lines tools to be installed. You must manually install the command line tools from within Xcode.
Run Xcode and goto the Xcode menu: Preferences > Downloads:
Select to install the Command Line Tools. Then a command like this will work:
PATH=$PATH:/Applications/Xcode.app/Contents/Developer/usr/bin && make -j 8