Fix few issues discovered by users of 1.4.0
- Added pkgconfig file to CMake install - Thanks @orbea for reporting and fixing (#1237 #1238).
- Fix regression in wchar support under windows. Thanks @Bak-Jin-Hyeong for reporting and fixing (#1239 #1240).
- CMake: Do not install bundled fmt if SPDLOG_FMT_EXTERNAL is defined. Thanks @orbea (#1241).
