Map Conan cppflags to CXXFLAGS in make generator

[danimtb]

Changelog: Fix: Map cpp_info.cppflags to CONAN_CXXFLAGS in make generator.
Changelog: Fix: Use *_DIRS instead of *_PATHS ending for varaibles generated by the make generator: INCLUDE_DIRS, LIB_DIRS, BIN_DIRS, BUILD_DIRS and RES_DIRS

Docs: conan-io/docs#1037

@tags: slow

• Refer to the issue that supports this Pull Request: closes Fix experimental make generator CONAN_CPPFLAGS and CONAN_INCLUDE_PATHS #4336
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[danimtb]

Not sure if this is a FIX or a BUGFIX, so reviewer please update the changelogs in the description if needed. Thanks! 😄