harden xsubpp locating loop in MM_Unix

While working on the WinCE interp build process, at one point xsubpp
wasn't in @INC or buildcustomize.pl, and EUMM silently continued and placed
a non-existant path into XSUBPPDIR in the makefile even though it never
found xsubpp.
Merge pull request #76 from bulk88/master

harden xsubpp locating loop in MM_Unix