Add CMAKE static library option

[burak-efe]

I couldn't figure out how to build as a static library, I guess is not possible from user side since there is no options to set unlike other libraries that I use. It would be good if it is exist.

[spnda]

Would you say just checking for the BUILD_SHARED_LIBS variable is enough, or should I introduce a extra flag like FASTGLTF_SHARED_LIB, since maybe only some libraries should be shared?

[burak-efe]

BUILD_SHARED_LIBS flag is enough for my use cases. But a special flag could be useful for some users indeed.