Use PCRE as built-in regex library

Replace the use of Henry Spencer's regex library with PCRE at the build
level and enable wxUSE_PCRE in wxRegEx code to switch to PCRE-based
implementation there.

Note that this has to be done unconditionally because there is no simple
way to select between the previously used regex library and PCRE at the
makefiles level.

We could still keep the possibility to use the system regex library
under Unix, but this doesn't seem to be worth doing, as we don't support
Unicode REs properly when using it (and never did), so drop support for
this too.

docs/changes.txt

@@ -12,7 +12,12 @@ INCOMPATIBLE CHANGES SINCE 3.0.x:
 Changes in behaviour not resulting in compilation errors
 --------------------------------------------------------
 
+- wxRegEx now uses PCRE library, changing the meaning of some exotic regular
+  expressions using wxRE_ADVANCED syntax, please see its documentation for more
+  details.
+
 - wxRibbonButtonBar::DeleteButton() now deletes and not just removes the button.
+
 - Default interpolation mode in wxGDIPlusContext under MSW is now
   wxINTERPOLATION_DEFAULT and not wxINTERPOLATION_GOOD as in 3.0 for
   consistency with OS X, call SetInterpolationQuality() explicitly if needed.