This should hopefully be useful as orientation for users implementing
the key exchange part of BIP324. Conceptually the example is not very
different to the ECDH one, so a lot of code/comments are just copied
(e.g. context creation, secret key generation, shared secret comparison,
console output, cleanup with secret key clearing).
This change simplifies running examples on Windows, because the DLL
must reside either in the same folder where the executable is or
somewhere in PATH.
It is a non-Libtool-specific way to explicitly specify the user's
intention to consume a static `libseck256k1`.
This change allows to get rid of MSVC linker warnings LNK4217 and
LNK4286. Also, it makes possible to merge the `SECP256K1_API` and
`SECP256K1_API_VAR` into one.
An executable target in the `COMMAND` option will automatically be
replaced by the location of the executable created at build time.
This change fixes tests for Windows binaries using Wine.
