fluent-bit: fix build on darwin
- Update fix-luajit-darwin.patch to apply to the current version.
- Add a patch to fix compilation of cmetrics on darwin.
- Use default stdenv instead of gccStdenv. The only requirement for GCC
was the bundled luajit having 'DEFAULT_CC = gcc' hardcoded, but it has
been fixed with the updated fix-luajit-darwin.patch by adding
DEFAULT_CC=cc to the arguments.
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