Compiling gtk libraries and gcc

[Marco Vittorini Orgeas <marco mavior eu>]

Hi,
I need to compile a software which requires a recent version of the GTK
library.
Being on debian stable, I am in the process of compiling from source the
gtk library which doesn't have a large set a of dependencies(just his
glib). My wish is to keep things well contained, so as suggested here
http://library.gnome.org/devel/gtk/unstable/gtk-building.html I am
compiling all the code under /opt (/opt/gtk /opt/glib).
When I try to compile the software which need those libraries I seem to be
just not able to tell gcc that it needs to compile against libraries under
/opt and not against the system ones under /usr/lib.

The strange fact is that the configure command execute without any
problems,but then when I issue the "make" command it complains about
"undefined reference to `gtk_orientable_set_orientation'"
sign that is using system old gtk library.

I tried also to export ,before issue the configure command, adjusted 
CPPFLAGS,LDFLAGS,PKG_CONFIG_PATH pointing them respectively to folders
under /opt/gtk/ /opt/glib etc...but it seems to have no effect (I read in
another post that: "LD_LIBRARY_PATH is  not needed when later running the
built software due to libtool's use of  rpaths").

So,what is the right way to tell gcc (make) to use one or another library
path when your system has two installed, one of which under /usr/lib and
so will probably be used by default ?

Also is there any recommended way to act in situations like that, when you
need a different version of a lib , and want to keep the system's one to
prevent breaking the system ? Put all in an isolate folder under /home or
/opt would be enough ?

At run-time are there any risks that the software choose the wrong .so
objects to load in memory from the system path? If it is compiled against
the right path, even if shared I think not....

Thank you for any clarification,

Keep me CC'ed please,

Marco Vittorini Orgeas