VS2019: get more of the targets to build (needed to specify modern C
standard for all of them, and SDK "10.0" seems to mean "use latest
version installed").
VS2019: improve README.md instructions.

Makefile: fix PEDANTIC=1 test builds and warning spam.

sys: properly fix warnings related to SDL's wrapping of main() for all
platforms.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>