* configure.in: Fix pkg-config detection of libxml2,
add -lgcrypt to gnutls lib.
* src/Makefile.am: If compiling --without-libvirtd then
don't compile any part of the storage driver.
* configure.in, src/hash.c, src/internal.h: Detect availability
of pthread library and compile out mutex code if not available.
* src/storage_driver.c, src/util.c: Ignore some missing
headers on MinGW.