Support gcc-11

[StrikerRUS]

Also, remove a line in README about rgf32.exe. We are no longer providing it.

[StrikerRUS]

@mlampros Hey! Could you please help with new CRAN NOTE on Linux that we didn't face earlier?

* checking for detritus in the temp directory ... NOTE
Found the following files/directories:
  ‘rgf’

Please feel free to push into this branch.

[mlampros]

hi @StrikerRUS. Let me have a look today afternoon to this new NOTE

[mlampros]

The NOTE mentions 'rgf' and 'rgf' is the root directory during checking. Would it help if we clean the temporary directory before we perform the check in the r_tests.sh file?

During my search based on the NOTE I found the following causes:
it might be a bug because it appears in the ubuntu-latest version (and we didn't encounter this error previously)
https://stat.ethz.ch/pipermail/r-package-devel/2020q4/006149.html
or somewhere in the scripts we start a web-browser or PDF viewer (as far as I can tell we don't do that)
either in the examples section
https://stackoverflow.com/a/62456524/8302386
or in the tests
https://stat.ethz.ch/pipermail/r-package-devel/2020q3/005701.html
or if there are any leftover files in the temporary directory
https://stackoverflow.com/a/62456199/8302386

[mlampros]

thanks

[StrikerRUS]

I've just uploaded a new version with this PR included
https://github.com/RGF-team/rgf/releases/tag/3.11.0
https://pypi.org/project/rgf-python/