[Build] Installed software missing libraries

[mboisson]

This started in the arrayfire-python repository, but this is actually more general. ArrayFire 3.9.0 does not install all required dependencies to its install directory, making it impossible to load some core ArrayFire shared libraries.

arrayfire/arrayfire-python#263

Libraries libspdlog.so.1 and libforge.so.1 are missing from the lib64 folder, making it impossible to load libafcpu.so and others.

[mboisson]

It seems like adding -DAF_BUILD_FORGE=ON installed libforge.so.1. This should be enabled by default if the built libraries are linked on that.

Additionally, I built using an external spdlog, which therefore has the .so already.