Move Ubuntu to gcc 9.2.1

[TokisanGames]

This merges against master.

I just built godot with both 32 and 64 bit dockers after this. There are few changed dependencies upon installing gcc 9. glibc is still 2.19.

Should not conflict with #40, but it will with #41 .

[akien-mga]

Thanks!

[akien-mga]

Note: I branched off the master branch from before this merge to 3.2, as the GCC version should stay pinned there for 3.2.x builds.

[akien-mga]

Actually I changed my mind and reverted this PR for now (well, force pushed master without this commit), as it was a hassle to have to handle syncing master and 3.2 once they have diverged with this commit, since I have a lot of work to do for 3.2 which is also needed for master (iOS C# support, Android and WebAssembly AOT compilation support for C#).

I'll re-apply this patch once I'm done with the 3.2+master common part, and once I start working on the master branch for 4.0 (we're still a long way away from the first alpha build).

[TokisanGames]

This was needed to build GD 4 previously, but now other changed requirements like scons 3 has rendered this docker unable to build GD 4 at all. I'm building 4 locally. We need to get off of trusty and move at least to 16.x, but that also bumps up our libc6 quite a bit.

[akien-mga]

Well we can still use Ubuntu 14.04, having Python 3 support in SCons is just a matter of changing the bogus shebang to point to /usr/bin/env python3 instead of /usr/bin/env python.

Then SCons requires Python 3.5 and Ubuntu 14.04 provides 3.4 by default, but it shouldn't be too hard to install 3.5 in parallel (I think I've seen instructions to do that).

[akien-mga]

This should give a functional scons in Ubuntu 14.04:

apt-get update
apt-get remove scons
apt-get install python3.5
curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
python3.5 get-pip.py
pip install scons

# Then:
python3.5 /usr/local/bin/scons
# Or:
sed -i /usr/local/bin/scons -e 's,/usr/bin/env python$,/usr/bin/env python3.5,'
scons

Installing pip via get-pip.py (since there's no python3.5-pip) is a bit overkill though, so it might be better to only install python3.5 and to get scons 3.1.2 via the scons-local upstream archive: https://scons.org/pages/download.html