buildsystem: Exporting CXXFLAGS and including them in the generated cmake toolchains.

[LeonardHerbst]

Contribution description

The CXXFLAGS are now also exported in vars.inc.mk.
The unwanted C++ flags (CXXUWFLAGS) and additional C++ flags (CXXEXFLAGS) were already being exported there, but the CXXFLAGS were missing.
Previously, the CXXFLAGS were set in Makefile.base and therefore could not be used by packages.
The CXXFLAGS are still constructed by filtering out the unwanted C++ flags from the regular CFLAGS and adding the additional C++ flags.
Since both of these variables are processed in cflags.inc.mk, it should be safe to construct the CXXFLAGS afterwards.

Testing procedure

The CXXFLAGS were used to generate the compile commands and compile C++ sources.
The CXXFLAGS are still included in the output of make compile-commands.
Both tests/build_system/cpp_ext and tests/build_system/cpp_exclude still pass.

[riot-ci]

Murdock results

❌ FAILED

218fa70 dist/tools/cmake: include CXXFLAGS in the generated toolchian

Success	Failures	Total	Runtime
325	0	9543	02m:00s

Artifacts

[crasbe]

Unfortunately moving the flag declaration like this can (and evidently has) unwanted side effects due to the grown nature of our make system :/

Apparently there is some flag missing at some place which leads to this (seemingly) unrelated build failure.
I can't really tell you which route would be the best way forward here tbh.

[LeonardHerbst]

The CXXFLAGS cannot simply be exported. They must dynamically reflect changes made to the CFLAGS (since they are constructed by filtering out CXXUWFLAGS and adding CXXEXFLAGS). This relies on the CXXFLAGS being evaluated lazily.

When make is called recursively, the CXXFLAGS are evaluated too early. The sub-make process only inherits the environment variable and therefore misses the flags added to the CFLAGS in the package/module/etc.