Add support for building static library with CMake

[ankane]

Hi, thanks for this library!

This PR adds support for building libb64c as a static library.

cmake -S . -B build -DBUILD_SHARED_LIBS=OFF
cmake --build build
# ... [100%] Linking C static library libb64c.a ...

It defaults to a shared library for backwards compatibility.

More info on BUILD_SHARED_LIBS: https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html

[jwerle]

thanks!

[ankane]

Thanks @jwerle!