Make all configuration options templates

All configuration options are now templates. The content of the rpm
spec file, and the build scripts are now configuration options which
by default use an include directive to include a template file.
Add the exit_error function in templates
README.md: fix typo
Add the exec function in templates

This function takes a command as argument, run it inside the sources
tree and return the output. The version option which was handled as a
special case in the source code to execute the command defined in
version_command if it is defined is now a normal option thanks to the
exec function.