Fix typo from too many cut-n-paste operations. Use IMPSRC:T to just
get the tail part of the path. We can now build kernels the
old-fashioned way on FreeBSD 9.x and 10.x on at least amd64 using
clang 3.3, 3.4 or gcc 4.2.1 (though with the latter you need
WITHOUT_MODULES="aesni vmm cxgbe" due to various issues with
gcc 4.2.1).