Add more "volatile" decls to stop gcc -Wall warnings.

Somebody w/ gcc please check that the wngs are gone!
There are cheaper (at runtime) ways to prevent the wngs, but
they're obscure and delicate.  I'm going for the easy Big
Hammer here under the theory that PCRE will be replaced by
SRE anyway.

Modules/pypcre.c

@@ -4520,9 +4520,9 @@ pcre_exec(const pcre *external_re, const pcre_extra *external_extra,
      they won't cost too much performance. */ 
 volatile int resetcount, ocount;
 volatile int first_char = -1;
+const uschar * volatile start_bits = NULL;
+const uschar * volatile start_match = (const uschar *)subject + start_pos;
 match_data match_block;
-const uschar *start_bits = NULL;
-const uschar *start_match = (const uschar *)subject + start_pos;
 const uschar *end_subject;
 const real_pcre *re = (const real_pcre *)external_re;
 const real_pcre_extra *extra = (const real_pcre_extra *)external_extra;