libglib2: disable tests and update configure options

Sometimes if host-python is around it doesn't work too well with the
distro python (Gentoo, FC20 at least) leading to build failures.

This failure is very hard to trigger, it's normally easier to do so by
enabling top-level parallel builds.

Fixes:
http://autobuild.buildroot.net/results/0f8/0f881e322b7f9d244b8dd2440ba0afd3a8418027/



While at it update the configure options to avoid any host
libraries/tools from leaking in, new naming conventions and options.
And remove dead code dealing with the nothreaded variant.

Signed-off-by: Gustavo Zacarias <gustavozacarias@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>