Specify each static library in full path instead of using the -Wl,-Bstatic trick.

This ensures that the correct library is linked independent of the
library search path, where previously the wrong libxml2.a might have
been picked if it had resided in the same directory as libruby.so for
example.
Revert c98745d9098487c51685a732f1c5a21d8d07cdad.

Let the ruby bug be there, as long as we make sure nokogiri will always
be linked with libxml2/libxslt/libexslt built from ports in the default
configuration.
Merge branch 'simplify_static'