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/readme.txt

@@ -148,9 +148,8 @@ If you use JPEG image handler, documentation for your program should contain
 following sentence: "This software is based in part on the work of the
 Independent JPEG Group". See src/jpeg/README for details.
 
-If you use wxRegEx class on a system without native regular expressions support
-(i.e. MS Windows), see src/regex/COPYRIGHT file for Henry Spencer's regular
-expression library copyright.
+If you use wxRegEx class, please see 3rdparty/pcre/LICENCE for PCRE licence
+details.
 
 If you use wxXML classes or XRC, see src/expat/COPYING for licence details.