meson/docs/markdown/snippets/preproc-flags.md

(C) Preprocessor flag handling

Meson previously stored CPPFLAGS and per-language compilation flags separately. (That latter would come from CFLAGS, CXXFLAGS, etc., along with <lang>_args options whether specified no the command-line interface (-D..), meson.build (default_options), or cross file ([properties]).) This was mostly unobservable, except for certain preprocessor-only checks like check_header would only use the preprocessor flags, leading to confusion if some -isystem was in CFLAGS but not CPPFLAGS. Now, they are lumped together, and CPPFLAGS, for the languages which are deemed to care to about, is just another source of compilation flags along with the others already listed.