Fix build with non-english locale

[moritz-h]

Description

The build fails on non-english systems due to parsing localized gcc output in CMake.
This was partially already fixed in #1450, but next to the LANG env variable also LC_ALL needs to be explicitly set to C otherwise LC_ALL or LANGUAGE can take precedence over LANG.
See gettext docs and GCC docs.

Type of change

• bug fix - change that fixes an issue
• new feature - change that adds functionality
• tests - change in tests
• infrastructure - change in infrastructure and CI
• documentation - documentation update

Tests

• added - required for new features and some bug fixes
• not needed

Documentation

• updated in # - add PR number
• needs to be updated
• not needed

Breaks backward compatibility

• Yes
• No
• Unknown

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