[Enhancement] Add option to build as shared libraries

[0x00002a]

This add the option MALLOY_BUILD_SHARED, which causes all libraries to be build as their shared versions (.dll on windows, .so on linux, etc). Since generator expressions cannot be used for add_library(.. [SHARED|STATIC]) I used an if statement and put it in a function (malloy_add_library).

Additionally, all shared libraries and binaries are now place in <builddir>/bin so stuff will actually run under windows. This is set on a per-target basis to avoid leaking into consumer builds. I think it is better to do this across the board rather than special case windows + shared configuration, since that introduces possible confusion and additional complexity. I am happy to change this though.

As discussed in #55 there might be issues with linking on windows, the CI seems to be fine though so hopefully its no longer a thing.

Closes: #55

[Tectu]

I don't really like the design with malloy_add_library(). What I would do is to have that MALLOY_BUILD_SHARED option exposed to the user and in the top-level CMakeLists.txt have a variable named MALLOY_LIBRARY_TYPE which gets set either to STATIC or SHARED (in the future I might add in OBJECT again) and then use that variable accordingly:

add_library(${TARGET} ${MALLOY_LIBRARY_TYPE})

This variable can then also be used for printing the build options so the user gets a nice text :)

[0x00002a]

I don't really like the design with malloy_add_library(). What I would do is to have that MALLOY_BUILD_SHARED option exposed to the user and in the top-level CMakeLists.txt have a variable named MALLOY_LIBRARY_TYPE which gets set either to STATIC or SHARED [..]

I didn't think of that, much cleaner :p