* 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.