[tlbuild] icu-76.2 in TL source tree, requires C++17

[Norbert Preining]

Github builds are now running again. Changes made:

- almalinux (building x86_64-linux)
  switch from gcc-toolset-9 to gcc-toolset-11
- freebsd
  bump -std from c++11 to c++17
- i386-linux
  switch from ubuntu:xenial to debian:buster
- alpine (building x86_64-linuxmusl)
  switch from alpine 3.5 to 3.12
- x86_64-linux (almalinux:8), i386-linux (debian:buster), x86_64-linuxmusl (alpine:3.12)
  add -std=c++17 to CXXFLAGS (supported but not on by default)

I guess some more fixes are necessary for the qemu builds (aarch64, arm)
which use buster, too, but we don't test that normally because it takes
ages.

Best regards

Norbert

--
PREINING Norbert                              https://www.preining.info
arXiv / Cornell University   +   IFMGA Guide   +   TU Wien  +  TeX Live
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