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.
There are plenty of unreleased variants of libsecp256k1 version 0.1.0
(libsecp256k1.so.0.0.0) in the wild. We choose a new version number to allow a
clear distinction.
There are variants of 0.1.0 that are incompatible with the initial release,
hence we increase the minor version to arrive at version number 0.2.0. For the
same reason, we increase the LIB_VERSION_CURRENT and keep AGE at 0.
The changelog for 0.2.0 consists of the relevant changes since 2021-12-25, which
is the date when the initial release process PR was merged (and the library
version was set to a pre-release, see 423b6d19d373f1224fd671a982584d7e7900bc93).
This is somewhat arbitrary but at least points readers to relevant changes.
