Code analysis functionality doesn't work on a cmake project, however works when clang-tidy is run manually

[Michał Zegan (webczat)]

Hi,
I have a cmake project (generally a C project, linux/gcc, using the libpipewire library).
When I run clang-tidy -p build somefile.c it works properly, as in, somefile.c comes from source directory, and build is a cmake build dir.
When I run code analysis from vscode, i get tons of errors, one per file, about errors in libpipewire header files. All these errors relate to some optional functions and stuff coming from locale.h.
For example:

call to undeclared function 'newlocale'; ISO C99 and later do not support implicit function declarations

I know that definition is introduced by defining some macros like _GNU_SOURCE, and if I actually do this, it starts working. However, normal cmake building and clang-tidy works without.
I think it's caused by the fact -std= is defined differently by default or in cmake builds, differently when running clang-tidy from vscode, as I can repro this when compiling any of these files and disabling language extensions.
Is this a cmake or c++ extension bug, or what?