Specify STATIC in add_library #234

I'm using Ryu from fast_matrix_market, but I think that the same applies also here.
In CMakeLists.txt, by calling add_library without the STATIC option, the choice between STATIC or SHARED is left to the BUILD_SHARED_LIBS option, set anywhere else in the parent CMake files.

However, since the CMake is currently not working with SHARED (at least on Win), thus implicitly restricting the build to STATIC only, might be worth to explicitly ask for STATIC?