Add back undefining _POSIX_C_SOURCE and _XOPEN_SOURCE

[mrbean-bremen]

• only undefining _POSIX_THREADS had been a problem
• see Revert undefining some defines #103

[he-hesce]

I think it's good!

[he-hesce]

Gold star release!

Integrates and passes integration tests in our in-house scientific interactive analysis software using Python as embedded scripting/extension language on:

• RHEL/CentOS 7: Qt 5.9, Python 2.7, gcc 4.8 - EOL 30 June 2027
• RHEL/Rocky 8: Qt 5.15, Python 3.6, gcc 8.5 - EOL 31 May 2031
• RHEL/Rocky 9: Qt 5.15, Python 3.9, gcc 11.3 - EOL 31 May 2034
• Ubuntu 20.04 LTS: Qt 5.12, Python 3.8, gcc 9.4 - EOL April 2025
• Ubuntu 22.04 LTS: Qt 5.15, Python 3.10, gcc 11.3 - EOL April 2027

[mrbean-bremen]

Thanks, good to know!

[he-hesce]

@mrbean-bremen: Although RHEL/CentOS 7 is our in-house platform and will slowly be moved to RHEL/Rocky 9 over the coming years. Having just managed a full/complete transition from RHEL 6 to RHEL 7 [1]. We skipped RHEL/Rocky 8 as it was just a possible upgrade path while waiting for 9. Ubuntu is just a backup platform in case RHEL/Rocky/AlmaLinux go (more than today) bonkers and we need a way out away from them - not that Ubuntu aren't bonkers themselves.

[1] Also means that use of C++11 is now approved by me and no need to support gcc 4.4 :-)