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/util.c b/util.c
index 0277deb..7d1af70 100644
--- a/util.c
+++ b/util.c
@@ -164,6 +164,7 @@
 	return NULL;
 }
 
+#if !defined HAVE_STPCPY
 char *
 stpcpy(char *dst, const char *src)
 {
@@ -171,6 +172,7 @@
 		dst++;
 	return dst;
 }
+#endif
 
 /*
  * Generic ptrace wrapper which tracks ESRCH errors