meson's configure_file emits only a comment like /* #undef ... */
for values which are unset in the configuration_data. For
gstglconfig.h, this differs from the autotools build where the
preprocessor definitions are always either 0 or 1. So loop over a
list of variables to set to zero as default.
Also sync up the gstglconfig.h.meson file with the additional
macros defined by the autotools build.
https://bugzilla.gnome.org/show_bug.cgi?id=781043