t/run/locale.t: Improve skip message

Better wording uggested by H.Merijn Brand

(cherry picked from commit 2ebcba0a9f172787c2c75bb3480e865c6809c1f0)
locale.c: Use PERL_UNUSED_RESULT

The previous way to suppress messages wasn't working for all gcc
versions.  Spotted by Jarkko Hietaniemi.

(cherry picked from commit 856b881c8af7c4319ce0cf631dae3495067185a2)