I found an unfortunate bug in the last patch I made to
patch-makefile-SHELL that would leave a trailing ' ' at the end of SHELL
assignments.  This is fine for most packages, but caused
gobject-introspection to fail building for me just now (for the curious:
it effectively does an "(apply system* (string-split (string-append SHELL
" " "./libtool") #\space))" which causes sh to try to execute "")

The attached patch to core-updates should remedy the problem.