[libvirt] [PATCH] Use SED variable for `sed` binary in src/Makefile

Hi, I've been trying to get libvirt (client) to cleanly/easily compile on BSD-based systems (in this case OS X). "./configure" runs fine but the "make" caused an error with `sed` since BSD sed was reporting some sort of regex error. I realized that the "SED" variable was populated with "gsed" which worked properly. This made the make continue (failed again later, will address that when I can). This is my first contribution to a C-based project and I don't have much experience with autotools other than using them as a consumer. Let me know if anything can be improved. Thank you, Mitchell