build: switch to POSIX make

[svmhdvn]

This change rewrites the main Makefile to use strictly POSIX make supported features. The main changes are:

• Use macro expansion as a means for portable conditional statements across all make implementations
• Use the POSIX accepted (and widely implemented across all make implementations) != shell command syntax
• Use POSIX compatible flags on shell commands

Tested on linux, freebsd, and macos using system provided make, CXX=g++, and CXX=clang++.

[svmhdvn]

Not sure why CI is failing on macos. I have tested independently with a friend's help on macos and it builds successfully. Also tested locally on both linux and freebsd, works fine.

[svmhdvn]

Pushed v2, this further cleans up the use of non-POSIX tools like install for simple POSIX equivalents. I may need to do one more iteration on this work, but it seems to be working for me locally.

Could you also figure out how to fix the CI pipeline so that these checks aren't failing based on monthly compute credits? Is there another provider you could use?

[mawww]

Any plans regarding the few inline comments ?

[svmhdvn]

Just pushed a version with a couple fixes that I missed. Are there any outstanding inline comments now? I think I've resolved them all.

[mawww]

Last missing thing is a copyright waiver commit, could you add that ?

[svmhdvn]

Done.

[mawww]

Thanks !