Support for precompiled headers with SPDLOG_ENABLE_PCH

Compilation is shorten by 2 on windows with msvc. Some improvement for gcc and appleclang. Add SPDLOG_ENABLE_PCH option to CMake that is defaulted to ON. This will enable precompiled header by default if supported. It can be disable if user have a conflict of any kind. Or to check that project is still compiling with CMake version that do not support precompiled header.

Fix Windows setenv check

README: add conda as a supported package manager

spdlog is available on conda-forge for a long time.
https://anaconda.org/conda-forge/spdlog