[libvirt] libvirt-php: various fixes

[Tiziano Mueller]

While building a libvirt-php package for Gentoo Linux I had to workaround a
couple of build issues. The first two patches are the results.

The last patch is a fix for the case libvirt has been built without Xen
support. In this case libvirt_connect (and phpinfo()) fails because
virGetVersion is called with a non-NULL typeVer argument and libvirt therefore
checks for Xen version.

I guess the documentation for virGetVersion should be updated since it also
returns 0 if both type and typeVer are NULL.

[PATCH 1/3] Fix building with threaded php.
[PATCH 2/3] add --with-php-config flag
[PATCH 3/3] fix libvirt_connect failure without Xen