fix-gnulib: Ensure header rules create output directory.

fix-gnulib: Ensure header rules create output directory.

Normally there is no problem, but when performing a VPATH builds
with --disable-dependency-tracking the "lib" directory does not get
created by configure and some of the rules that create header files
will fail.

It seems recent versions of Gnulib have a potential solution for this
particular problem but is easy enough to patch it up here too.