Change CI actions to a matrix with cmake options, for both release and
debug builds, with ENABLE_LINK=OFF and ENABLE_SDL=ON configurations, for
all 3 runners.
Also add both g++ and clang++ to the matrix for the Ubuntu runner to
test on clang.
Disable the macOS workflow for now because brew is failing to install
dependencies.
Signed-off-by: Rafael Kitover <rkitover@gmail.com>