configure: Try to use pkg-config to detect mbedTLS

configure: Try to use pkg-config to detect mbedTLS

mbedTLS does not seem to have pkg-config support on e.g.
Debian/Ubuntu, so we definitely need to keep the
fallback check as well.

Change-Id: I5d0da76018e874cda5dbab9202a2b817ad5e4692
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Yuriy Darnobyt <yura.uddr@gmail.com>
Message-Id: <20240906160510.76387-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29090.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>