Port to PCRE2

See https://lists.debian.org/debian-devel/2021/11/msg00176.html & https://pcre.org/

The older, but still widely deployed PCRE library, originally released in 1997, is at version 8.45. This version of PCRE is now at end of life, and is no longer being actively maintained. Version 8.45 is expected to be the final release of the older PCRE library, and new projects should use PCRE2 instead.

Close #11 (closed).

panel-plugin/Makefile.am

@@ -17,13 +17,13 @@ libverve_la_CFLAGS =	 						\
 	-DPACKAGE_LOCALE_DIR=\"$(localedir)\"				\
 	@LIBXFCE4PANEL_CFLAGS@						\
 	@LIBXFCE4UI_CFLAGS@						\
-	@LIBPCRE_CFLAGS@						\
+	@PCRE2_CFLAGS@							\
 	@GTHREAD_CFLAGS@						
 
 libverve_la_LIBADD = 							\
 	@LIBXFCE4PANEL_LIBS@						\
 	@LIBXFCE4UI_LIBS@						\
-	@LIBPCRE_LIBS@							\
+	@PCRE2_LIBS@							\
 	@GTHREAD_LIBS@
 
 libverve_la_LDFLAGS = \