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.
Remove opt_header_dirs which is obsolete.
Fix libiconv detection in cross building. (rake cross2 compile)
Make sure if the linker accepts -Wl,-Bstatic before using it.
Remove the ports build directory by default.
List available options for extconf.rb in a comment section.
Merge branch 'static_clean'