Use /usr/bin/gcc when making the OS X installer

On XCode 4.1, we use /usr/bin/gcc-4.2 as it makes better code than
/usr/bin/gcc for us. However, gcc-4.2 doesn't exist in XCode 4.2, so we
need to use /usr/bin/gcc there. As the installer can be used on either,
we make it always use /usr/bin/gcc.