Support for wolfSSL in OpenVPN

Support for wolfSSL in OpenVPN

This patch adds support for wolfSSL in OpenVPN. Support is added by using
wolfSSL's OpenSSL compatibility layer. Function calls are left unchanged
and instead the OpenSSL includes point to wolfSSL headers and OpenVPN is
linked against the wolfSSL library. The wolfSSL installation directory is
detected using pkg-config.

As requested by OpenVPN maintainers, this patch does not include
wolfssl/options.h on its own. By defining the macro EXTERNAL_OPTS_OPENVPN
in the configure script wolfSSL will include wolfssl/options.h on its own
(change added in https://github.com/wolfSSL/wolfssl/pull/2825). The patch
adds an option `--disable-wolfssl-options-h` in case the user would like
to supply their own settings file for wolfSSL.

wolfSSL:
Support added in: https://github.com/wolfSSL/wolfssl/pull/2503
```
git clone https://github.com/wolfSSL/wolfssl.git
cd wolfssl
./autogen.sh
./configure --enable-openvpn
make
sudo make install
```

OpenVPN:
```
autoreconf -i -v -f
./configure --with-crypto-library=wolfssl
make
make check
sudo make install
```

Signed-off-by: Juliusz Sosinowicz <juliusz@wolfssl.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20210317181153.83716-1-juliusz@wolfssl.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21686.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>