Do not search system directories so eagerly, but let user specify options.

Now that we use packaged libraries by default, Nokogiri should build
for most usrs without bothering system libraries.

We have the options --with-{xml2,xslt,exslt}-{dir,include,lib}=dir
supported and also consult pkg-config(1) for libxml2, libxslt and
libexslt.  What else should you want?
Link packaged libraries statically by default.

A new option --disable-static allows user to link packaged libraries
dynaminally.

Fix priority problems with include directories and library directories
by manipulating $CPPFLAGS, $LIBPATH and $libs correctly.
When this package is installed, clean ports files unnecessary to run.