Add support for static linking of `sqlx`

[emk]

sqlx requires linking against OpenSSL twice:

• sqlx-macros needs to link against OpenSSL as a shared libary for use at compile time.
• sqlx needs to link against OpenSSL a static library for use at runtime.

You can find the details here:

launchbadge/sqlx#670 sfackler/rust-openssl#1337

We go with the fix proposed by @sfackler, and add a test program that we can use to verify everything works correctly.

We remove a few OPENSSL-related variables that were added 4 years ago, and that no longer appear to be needed.