configure.ac: simplify AC_CHECK_FUNCS statements

configure.ac: simplify AC_CHECK_FUNCS statements

AC_CHECK_FUNCS checks availability of each function
in argument list and defines HAVE_function macro.
AC_CHECK_FUNC takes single function as an argument and
doesn't automatically define any macros.

When we check for availability of a single function and
define own macro, it is enough to use AC_CHECK_FUNC.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200121080828.1310-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19333.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>