Add --disable-sys-libs configure option.

This allows to disable the use of all system libraries with a single option
instead of having to use --with-{lib{png,jpeg,tiff},regex,zlib,expat}=builtin
or similar.

This is especially useful under OS X where we want to avoid the dependencies
on any locally installed homebrew or similar libraries.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76948 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775