Fix build on gentoo

References:
* https://bugs.gentoo.org/show_bug.cgi?id=466838
* https://github.com/brianmario/charlock_holmes/issues/10
Merge pull request #41 from vzctl/fix-gentoo-build

Fix build on gentoo
Use find_executable() instead of `which`.
Use icu-config if available.

Unfortunately, pkg_config() cannot be used here because it currently
only affects $CFLAGS and $LDFLAGS but not $CXXFLAGS that we need.

This fixes build on systems where ICU is installed in a non-system
directory.
Gemfile.lock is for apps, not for libraries.