changing --enable-gc-friendly=yes semantics

[Tim Janik <timj imendio com>]

as suggested earlier this week to matthias, i've now introduced
G_DEBUG=gc-friendly in CVS. this is compiled unconditionally into
GLib, so with the next development release, it'll be usable for 
all glib applications to 0-out released memory.

this changes the --enable-gc-friendly=yes semantics slightly,
instead of enabling the compilation of gc-friendly code portions,
it now changes the glib default for gboolean glib_mem_gc_friendly;
from FALSE to TRUE.

the changes are reflected in docs/macros.txt and in the online docs:
  http://developer.gnome.org/doc/API/2.0/glib/glib-running.html#G_DEBUG

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ciaoTJ