GMainContext useful in implementing some additional styles of main loop

GMainContext useful in implementing some additional styles of main loop

* gmain.c, gmain.h (g_main_context_new, g_main_context_destroy): GMainContext useful
in implementing some additional styles of main loop usage.  To do this, however, Joe
Hacker needs to be able to create/destroy GMainContext's at will. This is just an
export of existing functionality, rather than new functionality.

They are listed in the "Low level functions for implementing custom main loops"
section of the header file, to avoid confusing people.

* gobject/Makefile.am:
  . You have to 'touch oldest-source-stamp' if you want to avoid having
  the Makefile constantly rebuild itself.
  . Fix marshaller generation rules to work with srcdir != builddir
    (there were issues with trying to run "./glib-genmarshal", etc.)