plugins: no need to redeclare environ

From the manpage :

    This variable must be declared in the user program, but is declared in the
    header file unistd.h in case the header files came from libc4 or libc5, and
    in case they came from glibc and _GNU_SOURCE was defined
plugins: environ has to be declared on non-gnu
Merge branch 'master' of github.com:munin-monitoring/munin-c
travis: initial add