Consider storing openssl prefix in RbConfig::CONFIG["configure_args"]

[eregon]

See ruby/openssl#650 (comment)

The current approach taken in TruffleRuby is similar to what happens on CRuby when using --with-opt-dir, like ruby-install does it:
https://github.com/postmodern/ruby-install/blob/f4978056b54f6926b5c46d86aae19a90544bdd60/share/ruby-install/ruby/functions.sh#L39

[eregon]

Note: we should be able to revert ruby/openssl#653 when this is fixed.

[eregon]

So interestingly #3174 as it is breaks installing grpc:

./src/core/tsi/ssl/key_logging/ssl_key_logging.h:23:10: fatal error:
'openssl/ssl.h' file not found
#include <openssl/ssl.h>
         ^~~~~~~~~~~~~~~

Because now the include dir of openssl is no longer in cppflags, and grpc extconf.rb doesn't call dir_config("openssl"), and on TruffleRuby grpc doesn't build its own openssl (notably saves a fair amount of build time + other concerns):
https://github.com/grpc/grpc/blob/44864c1589040d5dfcb9c793a070efe51d0fb574/src/ruby/ext/grpc/extconf.rb#L70-L71

We could set both configure_args and cppflags/... but that feels a bit wrong.
Or we could make a PR to use dir_config("openssl") in grpc, that might take a while to get merged & released though.
WDYT @rhenium?

[eregon]

This will be fixed by #3343.