MAkefile support and compiler warnings

[brianwalenz]

I split the existing CXXFLAGS into multiple lines, added a few -Wno options, and removed redundant -lpthread from the link. The rest of the changes are pretty simple.

Compilation:

• Linux RHEL 8.9 is clean.
• MacOS has not been tested.
• FreeBSD 13.2 (gcc9) is clean.
• FreeBSD 13.2 (clang14) still reports:

  src/HashList.h:47:9: warning: private field 'kmerSize' is not used [-Wunused-private-field]
          size_t kmerSize;
                 ^
  1 warning generated.
  

Execution has NOT been tested yet. No need to merge until it is tested, and I guess MacOS too. MacOS arm is known broken, correct?