Mac support

[MaddTheSane]

Initial Mac support.
Based on the work of @almeath and @kode54 at https://github.com/PCemOnMac/PCemV17macOS

[sarah-walker-pcem]

Changes to SDL #include breaks the Windows build.

[Shambler2]

Have done some testing on the other project when they were attempting to add M1 support on macOS - one of the changes needed for that support doesn't appear to be in this pull request, configure.ac requires some minor changes to correctly identify the M1 as an arm CPU:
AC_MSG_CHECKING([for cpu]) case "${host_cpu}" in arm) CPU=arm64 # MacOS has "arm" in the host string AC_MSG_RESULT(${host_cpu}) ;;

[MaddTheSane]

I know.
I don't want to use that override due to the fact that it might confuse checks on 32-bit ARM machines.
The check will need to be more advanced than @almeath's way.
I got around this by passing --build=aarch64-apple-darwin20.4.0 to configure. This isn't ideal, I know.

[Shambler2]

Yep figured that might have been the case :)

Not ideal but thanks for the workaround for now