Updates supporting CPack RPM generation

Enable command line overrides of default settings
* CPACK_GENERATOR
* CPACK_PACKAGE_RELOCATABLE

Enables command line setting of generated RPM version attributes
* CPACK_RPM_PACKAGE_RELEASE
* CPACK_RPM_PACKAGE_ARCHITECTURE

Adds dependency on pkgconfig RPM when not generating relocatable RPM

Add example to FMT_STRING in README

Fix Windows event log sink

workaround for `Unknown extension ".c" for file` issue

http://www.grokit.ca/cnt/CMakeProblemsSolutions/

Avoid references to race data filename

Add critical section to filename function of  each file sink

Handling SPDLOG_PREVENT_CHILD_FD in tcp_sink

Adding SOCK_CLOEXEC to socket

Fixing bug in sink_it_ (bytes_sent not added to buffer)