switch off locale "C" override for opencl compilation by default

[dterrahe]

fixes #18196

This does two things:

• by default, it no longer switches to the "C" locale while compiling opencl. However this can be reenabled by adding "opencl_force_c_locale" to darktablerc
• even when the override is in place, it switches to "C" in far fewer cases and shorter, namely only when actually compiling. So if the workaround is still needed, it would only impact the first startup after program or driver updates and maybe not even break anything there.

I have not tested this beyond that it compiles and runs, because I cannot easily replicate the original issue to verify this fixes it.

RELEASE NOTE:

• A workaround for old broken AMD opencl drivers that has been in place for 12 years has been disabled by default. Hopefully more recent drivers don't need it anymore and it caused problems with the translation of the user interface during startup, causing part of it to appear in English.
  If you have problems with OpenCL that you believe may be caused by this change, please do two things:
  1. Add the line opencl_force_c_locale=anything to your ~/.config/darktable/darktablerc file.
  2. If that solves your problem, please report the issue to github PR #xxxx, including information on the make of your GPU and the version of your drivers. We can then for the next release either by default reenable the workaround for drivers that need them, or warn people with antiquated drivers that they need to update.

[TurboGit]

All good to me, we can also later only do that if AMD is in the list of OpenCL driver.

[rgr59]

Tested this PR and cannot reproduce #18196 anymore. Thanks!