Catch2 fails to compile on Fedora Rawhide (more recent version needed)

[jeff-cohere]

Describe the bug
One of the EAGLES/HAERO developers encountered an issue described here. This issue has been fixed downstream, so it would be nice to get a more recent version of the Catch2 header into EKAT.

To Reproduce
Steps to reproduce the behavior:

1. Log into a system running Fedora Rawhide (or another system with a particularly picky constexpr-obsessed C++ compiler?)
2. Try to build EKAT
3. See error

Here's what it looks like for us:

In file included from /usr/include/signal.h:328,
                 from /home/asher/workspace/haero/ext/ekat/extern/catch2/include/catch2/catch.hpp:7712,
                 from /home/asher/workspace/haero/ext/ekat/src/ekat/util/ekat_catch_main.cpp:3:
/home/asher/workspace/haero/ext/ekat/extern/catch2/include/catch2/catch.hpp:10453:58: error: call to non-‘constexpr’ function ‘long int sysconf(int)’
10453 |     static constexpr std::size_t sigStackSize = 32768 >= MINSIGSTKSZ ? 32768 : MINSIGSTKSZ;
      |                                                          ^~~~~~~~~~~
In file included from /usr/include/bits/sigstksz.h:24,
                 from /usr/include/signal.h:328,
                 from /home/asher/workspace/haero/ext/ekat/extern/catch2/include/catch2/catch.hpp:7712,
                 from /home/asher/workspace/haero/ext/ekat/src/ekat/util/ekat_catch_main.cpp:3:
/usr/include/unistd.h:640:17: note: ‘long int sysconf(int)’ declared here
  640 | extern long int sysconf (int __name) __THROW;
      |                 ^~~~~~~

Expected behavior

No compile errors, please!

Desktop (please complete the following information):

$ uname -a
Linux fedora 5.14.10-300.fc35.x86_64 #1 SMP Thu Oct 7 20:48:44 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Additional context
Add any other context about the problem here.

[bartgol]

I was actually thinking we should move to a submodule version of catch2, rather than copy-paste their all-in-one header.

[jeff-cohere]

Hopefully this is fixed by #185. I'll have Asher check this when we've got EKAT updated within Haero.