tools: use pkg-config to found libgc

tools: use pkg-config to found libgc

In some platform (macosx+macports), the libgc library is not located at
standard default position.
Fortunately, libgc seems to be registered with pkg-config under most
platforms.

Thus, use pkg-config in the generated makefiles to found the correct
flags and paths.

In c_src, some generated files are manually modified to provide a
working bootstrap under those platforms.

This is only a quick-and-dirty workaround. The real job must be polished
and generalized to solve the issue #864.

Reported-by: Alexandre Blondin Massé <alexandre.blondin.masse@gmail.com>
Signed-off-by: Jean Privat <jean@pryen.org>