package/spdlog: support dynamic library

spdlog can install a shared library since version 1.4.0 and
https://github.com/gabime/spdlog/commit/45a18a61c633564311e51f5cbed2748e9cb70e2f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>