Makefile: Add README to distributed files

README was accidentally omitted from distribution, and the link from
README to README.md was missing since the file was renamed with the
Markdown extension.
contrib: Do not generate RPM spec

The RPM spec needs to be maintained manually anyway so it does not make
sense to autogenerate the file with the latest version number. This should
be done manually while updating dependencies and changelog.
systemtime: Use gettimeofday if POSIX timers not available