build: fix broken mingw cross-compilation

build: fix broken mingw cross-compilation

Two regressions:
Commit df1011ca broke builds for systems that lack devmapper
(non-Linux, as well as Linux with ./autogen.sh --without-libvirtd
and without the libraries present).
Commit ce6fd650 broke cross-compilation, due to a gnulib bug.

* .gnulib: Update to latest, for cross-compilation fix.
* src/util/util.c (virIsDevMapperDevice): Provide stub for
platforms not using storage driver.
* configure.ac (devmapper): Arrange to define HAVE_LIBDEVMAPPER_H.
devmapper issue reported by Wen Congyang.