build: avoid passing empty strings to build flags

While checking the return values from icu-i18n, we didn't
validate the content before passing it to the build system.

Also make cflags parsing more robust by avoiding empty strings.

Fixes: https://github.com/nodejs/io.js/issues/1787
PR-URL: https://github.com/nodejs/io.js/pull/1789


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>