package/mutt: do not run compiled programs from ./configure

mutt's ./configure is broken for cross-compilation, because it runs
programs to check for some features or behaviours.

So, we have to feed it some variables to force the checks the way they
are supposed to be resolved for our use-case.

Fixes:
    http://autobuild.buildroot.net/results/58c/58c73e72e1387c7f490ad97a86666b12393917cf/
    http://autobuild.buildroot.net/results/73b/73b8c54ca41cc475594c24a8d6176d511dcb85e6/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>