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.
