[configure] add configure support for PCRE Posix library

Debian tried unsuccessfully to build with libpcreposix.  This adds proper
support to autoconf/configure.ac for it.

Based on the patch by C.J.  Adams-Collier in bug #483.

bgpd/bgp_{regex,routemap}.h: Pull in PCRE POSIX header if available.
diff --git a/bgpd/bgp_regex.h b/bgpd/bgp_regex.h
index 176b65a..9fc8322 100644
--- a/bgpd/bgp_regex.h
+++ b/bgpd/bgp_regex.h
@@ -23,11 +23,15 @@
 
 #include <zebra.h>
 
-#ifdef HAVE_GNU_REGEX
-#include <regex.h>
+#ifdef HAVE_LIBPCREPOSIX
+# include <pcreposix.h>
 #else
-#include "regex-gnu.h"
-#endif /* HAVE_GNU_REGEX */
+# ifdef HAVE_GNU_REGEX
+#  include <regex.h>
+# else
+#  include "regex-gnu.h"
+# endif /* HAVE_GNU_REGEX */
+#endif /* HAVE_LIBPCREPOSIX */
 
 extern void bgp_regex_free (regex_t *regex);
 extern regex_t *bgp_regcomp (const char *str);