fix: build shared fmt libraries

[c-dilks]

See fmt build option documentation:
https://github.com/fmtlib/fmt/blob/bf98e3e4c61a7f5c36633278da6567f7bcedd4d5/doc/usage.rst?plain=1#L48-L60

Resolves #75 no, this should be a fallback if static libfmt doesn't exist

cppyy on macOS seems to rely on the shared (dynamic) fmt library, which wasn't being built on the CI or recommended in the documentation. In fact, the Homebrew fmt formula builds both the static and dynamic libraries (whereas, e.g., Arch Linux only builds the shared library.

[c-dilks]

Perhaps we close this PR, since:

• systems which only have shared fmt libs should work fine, since they will be found in the default locations (no rpath additions needed)
• users who build fmt themselves should build the static library, not the shared library
• macOS users who want to use the Python bindings will have to build both, since for some reason cppyy on macOS requires the shared libraries; this is what its Homebrew formula does