gpp-2.28-bp156.1.3

- Cleanup spec file with spec-cleaner
- Add gpg signature

- Improved compliance with openSUSE spec file guidelines

- Initial build

- update to GPP 2.28
  * Fixed typos in documentation (Issues #57 and #61)
  * Added #sinclude meta-macro for silent includes (Issue #63)
  * requested inclusion in Leap 15.4 and 15.5; see
    https://bugzilla.suse.com/show_bug.cgi?id=1108673

- update to GPP 2.27
  * --help and --version now print to stdout, not stderr (Issue #47)
  * Various minor fixes and revisions to the documentation (Issues #38
    through #44)

- update to GPP 2.26
  * Made --include work when using a relative path to the infile
    (Issue #16)
  * Various minor fixes and revisions to the documentation
  * Added autoconf checks for popen() and pclose()
  * Moved the openSUSE-specific RPM spec file to a separate repository
  * Avoided possibility of stack overflow during parsing (Issue #26;
    CVE-2018-17076)
- update keyring