maint: port to Autoconf 2.71

maint: port to Autoconf 2.71

* configure.ac: Use AC_PROG_CC, not AC_PROG_CC_STDC.
* gl/modules/smack (configure.ac):
* m4/jm-macros.m4 (coreutils_MACROS):
* m4/xattr.m4 (gl_FUNC_XATTR):
Use AS_HELP_STRING, not AC_HELP_STRING.
* m4/check-decl.m4 (gl_CHECK_DECLS):
Do not require AC_HEADER_TIME; we no longer care about it directly.
* m4/jm-macros.m4 (coreutils_MACROS):
Do not require AC_ISC_POSIX, which became obsolete in 2006.
Use AC_LINK_IFELSE instead of AC_TRY_LINK.