Add stpcpy to autoconf machinery

* configure.ac: Add stpcpy to AC_CHECK_FUNCS.
* defs.h: Frame stpcpy with "if !defined HAVE_STPCPY".
* util.c: Likewise.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/defs.h b/defs.h
index 59d4c7a..dd8fc64 100644
--- a/defs.h
+++ b/defs.h
@@ -681,12 +681,14 @@
 extern void tv_mul(struct timeval *, struct timeval *, int);
 extern void tv_div(struct timeval *, struct timeval *, int);
 
+#if !defined HAVE_STPCPY
 /* Some libc have stpcpy, some don't. Sigh...
  * Roll our private implementation...
  */
 #undef stpcpy
 #define stpcpy strace_stpcpy
 extern char *stpcpy(char *dst, const char *src);
+#endif
 
 #ifdef SUNOS4
 extern int fixvfork(struct tcb *);