dnl @synopsis AC_CHECK_ENVVAR
dnl
dnl This macro tests if a particular environment variable is defined.
dnl Warning if not.
dnl
dnl Example:
dnl
dnl     AC_CHECK_ENVVAR(foo)
dnl

AC_DEFUN([AC_CHECK_ENVVAR],[dnl
AC_MSG_CHECKING([value of $1])
if test "x@S|@$1" != "x"; then
	AC_MSG_RESULT([@S|@$1])
else
	AC_MSG_WARN([not defined! (please fix)])
fi
])