Set additional CPACK variables for RPM generator

Sets the following when querying the generated RPM's info:

Group       : System Environment/Libraries
License     : MIT
URL         : https://github.com/gabime/spdlog
Summary     : Fast C++ logging library
Description :
Very fast, header-only/compiled, C++ logging library.

cmake: Only install fmt headers when SPDLOG_FMT_EXTERNAL is not defined.

cmake: Add a new spdlog.pc pkgconfig file.

Fix #1239, Remove basic_string_view_t

Fixes https://github.com/gabime/spdlog/issues/1237.

Unbreak on more BSDs

Modern operating systems don't need to implement transitional
extensions for large file support.