Adding fix for building GNU m4 on Debian

[Hubert Feyrer <hubert%feyrer.de@localhost>]

Trying to build pkgsrc/devel/m4 on Debian gives me an error:

        ...
        gcc -std=gnu99  -I.      -O2 -Wno-error -MT execute.o -MD -MP -MF
        .deps/execute.Tpo -c -o execute.o execute.c
        In file included from execute.c:47:0:
        ./spawn.h:427:3: error: conflicting types for 
'posix_spawn_file_actions_t'
        /usr/include/spawn.h:50:3: note: previous declaration of
        'posix_spawn_file_actions_t' was here
        execute.c: In function 'execute':

I have a quick fix for this, but am not sure how to really use this so 
it's only applied to Debian / where needed (i.e. not on OS X):

        # Fix building on Debian Linux,
        # see 
http://lists.freebsd.org/pipermail/freebsd-ports/2009-July/056108.html
        CONFIGURE_ENV=ac_cv_func_posix_spawn=yes

How do I best add this to the pkg?


 - Hubert