regex-tests, regex: fix bug: memset undeclared

* tests/test-regex.c: Don't include regex.h twice.  Include
string.h, to declare memset.  Christensen's report also mentioned
this issue.
* m4/regex.m4 (gl_REGEX): Keep test program more in sync with
test-regex.c, to avoid future problems like this.  Remove
AC_INCLUDES_DEFAULT.  Include <string.h>.  Don't include <regex.h>
twice.