Added support for gnu-make $(file when available

[Norbert Thiebaud]

Recently on gnu-make cvs a new function has been added: $(file
>filename,content)
This allow to put the content on a file natively (without using the
shell, and therefore there is no command line limitation
This can be used to simplify $(var2file, whihc I have long suspected
to be the reason why MacOSX is occasionally failing with 'Too Many
Open Files' errors.

I've pushed 2 patches. one add the detection of that feature in
gnumake during configure, the other use a simplified $(var2file when
that feature is available

I've also back-ported the feature in gnu-make 3.82 in contrib/dev-tools


Norbert