Linux build fails against system libwebsockets using OpenSSL

[akien-mga]

Godot version:
3.1-stable
f64336f

OS/device including version:
Mageia 7 64-bit
Packages:

• lib64websockets-devel-3.1.0-1.mga7 (RPM .spec file)
• lib64openssl-devel-1.1.0j-1.mga7 (pulled in as dependency of libwebsockets)

Issue description:
Building with scons p=x11 builtin_libwebsockets=no on above system fails with:

modules/websocket/lws_client.cpp: In member function 'virtual int LWSClient::_handle_cb(lws*, lws_callback_reasons, void*, void*, size_t)':
modules/websocket/lws_client.cpp:131:59: error: cannot convert 'const unsigned char*' to 'const unsigned char**'
     SSL_CTX_add_client_CA((SSL_CTX *)user, d2i_X509(NULL, &arr.read()[0], arr.size()));
In file included from /usr/include/openssl/objects.h:916,
                 from /usr/include/openssl/evp.h:27,
                 from /usr/include/openssl/x509.h:23,
                 from /usr/include/openssl/ssl.h:50,
                 from /usr/include/libwebsockets.h:223,
                 from modules/websocket/lws_peer.h:39,
                 from modules/websocket/lws_helper.h:41,
                 from modules/websocket/lws_client.h:37,
                 from modules/websocket/lws_client.cpp:33:
/usr/include/openssl/x509.h:540:1: note:   initializing argument 2 of 'X509* d2i_X509(X509**, const unsigned char**, long int)'
 DECLARE_ASN1_FUNCTIONS(X509)
 ^~~~~~~~~~~~~~~~~~~~~~

Seems like a conflict between a system-provided libwebsockets + openssl compared to our builtin libwebsockets + mbedtls.

Steps to reproduce:

• Install distro package for libwebsockets
• scons p=x11 builtin_libwebsockets=no