Enable nacl_io in pepper-ruby.

* configure.in (XCFLAGS): Add include path for NaCl libraries.
  (XLDFLAGS): ditto.
  (NACL_LIB_PATH): new stubstitution

* nacl/nacl-config.rb: support NACL_LIB_PATH

* nacl/package.rb: ditto.

* nacl/pepper_main.c: replace old implementations with nacl_io.

* nacl/GNUmakefile.in: link nacl_io to pepper_ruby

* ruby.c (rb_load_file): remove __attribute__((weak)) because the old
  override hack was replaced with nacl_io.

* file.c (rb_file_load_ok): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48078 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* remove trailing spaces.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48079 b2dd03c8-39d4-4d8f-98ff-823fe69b080e