lib: Reduce dependency on glib

In commit ebd6eb69f57b ("Make igtrc configuration common, with
configurable suspend/resume delay") .igtrc handling was moved to
igt_core from igt_chamelium. That made everything in IGT depend on
GLIB by accident.

In short, igt_core.h declared a variable of type GKeyFile*, requiring
glib.h. Everything that tried to #include igt_core.h required glib.h
to be available, by use of GLIB_CFLAGS. This "worked" so far because
CAIRO_CFLAGS contained GLIB_CFLAGS.

As the variable is only used by other stuff in lib/, stuff it in its
own header file to leave igt_core.h without a dependency to glib.h.

Also add a couple of missing #ifdef HAVE_GLIBs around.

Reported-by: Felipe De Jesus Ruiz Garcia <felipe.de.jesus.ruiz.garcia@intel.com>
Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
6 files changed