On Debian/Ubuntu, one of the libraries libvirt (indirectly) links
with exports a symbol named 'base64_encode'. This takes precedence
over GNULIB's base64_encode function during linking. Unfortunately
they of course have different API semantics. To avoid this problem
use a few #defines in config.h to rename the GNULIB provided
function to have a 'libvirt_gl_' prefix