[bug] AutoToolsDeps sets includes to CPPFLAGS, but MesonToolchain reads only CFLAGS/CXXFLAGS?

[SSE4]

related to #10683, coming from conan-io/conan-center-index#11053

in an attempt to migrate Glib to MesonToolchain I had to put the following workaround:

        cppflags = tc.vars().get("CPPFLAGS")
        tc.environment.append('CFLAGS', cppflags)
        tc.environment.append('CXXFLAGS', cppflags)

to propagate CPPFLAGS into CFLAGS/CXXFLAGS.
AutoToolsDeps sets includedirs to CPPFLAGS only:
https://github.com/conan-io/conan/blob/1.48.2/conan/tools/gnu/autotoolsdeps.py#L46
https://github.com/conan-io/conan/blob/1.48.2/conan/tools/gnu/autotoolsdeps.py#L74
while MesonToolchain currently only reads from CFLAGS/CXXFLAGS:
https://github.com/conan-io/conan/blob/1.48.2/conan/tools/meson/toolchain.py#L154-L157

so they kinda cannot connect to each other as is, which is disappointing. I don't know where shoud be the right fix, in MesonToolchain or AutoToolsDeps.

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[jcar87]

According to: https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html#Implicit-Variables

• CXXFLAGS are for the C++ compiler
• CFLAGS for the C compiler
• CPPFLAGS are for the C and Fortran preprocessor - that said, this is a bit confusing - because most flags that control the preprocessor https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html - can be passed directly to the compiler as far as I'm aware

Meson only listens to CXXFLAGS (similar to cmake) - but they recommend not using these: https://mesonbuild.com/Reference-tables.html#compiler-and-linker-flag-environment-variables