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

docs/changes.txt

@@ -37,6 +37,10 @@ All:
 - Define wxOVERRIDE as override for supporting compilers (Thomas Goyne).
 - Allow specifying custom comparator for wxSortedArrayString (Catalin Raceanu).
 
+Unix:
+
+- Add --disable-sys-libs configure option.
+
 All (GUI):
 
 - XRC handler for wxAuiToolBar added (Kinaou Hervé, David Hart).