Various build updates

[d-torrance]

• Remove some unnecessary build flags:
  • C++17 is now handled in the configure script, so we don't need to do it in the Makefile
  • automake already takes care of adding the -I flags for the source directory, so we don't need to worry about adding it. Plus CXXFLAGS is the wrong variable for that anyway.
  • Set the I$(srcdir)/src build flags in configure
• Allow running autogen.sh from the build directory
• Update the GitHub workflow -- cmake & macOS builds now supported
• Go back to hard-coding the version number since the M2 cmake build only uses the src directory.