Add YouCompleteMe support

YouCompleteMe[1] is a vim plugin that implements semantic
code completion using libclang.

For non-trivial projects such as libvirt, the plugin needs
some help figuring out where to find the various header
files: generate its configuration file at configure time
so that the plugin works out of the box.

[1] http://valloric.github.io/YouCompleteMe/

configure.ac

@@ -883,6 +883,7 @@ AC_CONFIG_FILES([\
         gnulib/lib/Makefile \
         gnulib/tests/Makefile \
         .color_coded \
+        .ycm_extra_conf.py \
         libvirt.pc \
         libvirt-qemu.pc \
         libvirt-lxc.pc \