Improve CMake code

[ndam-hexagon]

This PR modernises fmtlog's CMake code:

• Allow building using a version of fmtlib found on the system (by default use the local version)
• Use BUILD_TESTING to control whether to enable or disable tests
• Properly install the project using a CMake installation config file
• Cleanup various other aspects

[josephholten]

I am using this branch currently as a git submodule in my repo which I build with cmake and add_subdirectory. the master branch requires me to link myself with fmt::fmt which is not as elegant as this PR.

Would love to see this PR merged! Great lib BTW.