build(nanobind): Set NB_BUILD and NB_SHARED macros for libnanobind

[nicholasjng]

This way, libraries can consume symbols from dynamically linked libnanobinds, i.e. shared library builds of nanobind.

Closes #81. This hopefully also fixes LLVM build failures seen in llvm/llvm-project#169306, when released on the BCR.

Maybe someone with more experience can explain if these definitions impact static linkage of nanobind.

[wjakob]

NB_SHARED enables DLL linkage for exported libnanobind symbols. When doing a shared build, you must set NB_BUILD to export symbols, otherwise the default is that they are imported. This is especially important for windows where the symbol export/import direction must be specified for each symbol. When doing static builds NB_SHARED should not be specified. The NB_BUILD flag is ignored if NB_SHARED is not set.

[nicholasjng]

Thank you! That helps.

I'm not sure if I can control defines passed to a cc_library based on its linkage, or if a cc_library target can have multiple linkages, but both JAX/XLA and LLVM set these unconditionally, and they seem to be fine. I'll ask around.