Really fix installing Ruby 2.1.0 from trunk

The problem wasn't in quoting as per 0b5206172, but in the fact that
Ruby trunk added a LDFLAGS checker that aborts if any of the paths
listed in it are missing:
https://github.com/ruby/ruby/commit/3636f8c0f56ddf15e26e28e7a38e748588fad976

This is probably a bug in Ruby, but for now a simple workaround is to
iterate through paths in LDFLAGS and ensure they exist.

References #441