Missing find_package for json dependency

[Jarnovr]

Hi,

I'm building Thermofun 0.3.8 from sources on windows (as part of building reaktoro).
The build is failing as the source files reference nlohmann/json.hpp which they can't find.

In ThermoFun\CmakeLists.txt around line 40 there is some conditional code and on windows with visual studio nothing is done while on linux there is a dependency explicitly defined.

I added this to the MSVC section and that worked:

   find_package(nlohmann_json REQUIRED)
		 target_link_libraries(ThermoFun
         PRIVATE nlohmann_json::nlohmann_json)