correct/improve/simplify makefile unit test

[solvingj]

Changelog: Fix: Improve make generator test

This is a followup adjustment to:
#4003
which closed:
#3773

The makefile generator unit test still had some small issues, summarized as they appear below. On one hand, the builds and tests passed, making these seem like mostly cosmetic issues. On the other hand, people sometimes look at tests, and probably moreso for new generators than anything else. Non-gcc experts might be confused about how they should use these flags/vars in their own makefiles, and gcc-experts might be concerned that the unit test does things incorrectly and still passes. Best to make these test as correct as possible.

-fPIC/-fPIE are particularly important flags, which need to go to both compiler and linker, and should always be set by the current package, never propagated from upstream dependencies.

-march/-mtune would be defined on "platform" basis, likely passed in via a profile/environment_varaible/toolchain file rather than from upstream dependencies.

-Thus, replaced the cflags above with ones that people might realistically want to pass from upstream to downstream. Of note, -pthread is indeed a compiler flag separate from the linker flag -lpthread.

-Modified CXX_INCLUDE_PATHS var name to be consistent with CONAN_INCLUDE_PATHS

-Added new CFLAGS variable and set it equal to $(CONAN_CFLAGS). It turned out that the latter was simply ignored (never referenced) in the build, and that was definitely a problem. This is probably the most significant of the improvements.

-Simply removed the use of the CXX_FLAGS variable. There was no reason to add a new variable, as it just complicates the example overall. Also, this removes the -fPIC compiler flag from the equation completely, which is good because it's not strictly necessary. To handle fPIC and fPIE properly, one should make it a Conan option and only apply it conditionally in the makefile, none of which is necessary for this test.

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[danimtb]

Thanks @solvingj for the explanation. I think having a more realistic test is always a good thing.

Having a look at the variables I fail to see the same logic as described here: https://docs.conan.io/en/latest/reference/build_helpers/autotools.html#environment-variables

Which I think is the right convention as I explained with the table in here: conan-io/docs#955 (comment)

We need to reach an agreement or probably look more into it

[danimtb]

Also note that the hellowrapper recipe requires a hello one compiled with CMake that should be setting automatically the fPIC flag and the test in CI is failing