build: use gnulib's unsetenv

Now that gnulib has lifted it's licensing of unsetenv, we should
use it. Just like we use its counterpart - setenv, already.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

bootstrap.conf

@@ -120,6 +120,7 @@ time_r
 timegm
 ttyname_r
 uname
+unsetenv
 useless-if-before-free
 usleep
 vasprintf