Updating from easylogging++ 9.89 to 9.96.7 turns it from a header only
library to one requiring a source file. It has been added to sources
near the other third party includes in the QMAKE to signify that it
isn't a first party file.
Updating easylogging++ also means that clang-msvc can compile without
any warnings, hence turning on warnings as errors for that compiler as
well, to avoid any regressions.
