Switch MinGW building to CMake

Automatically grabs and compiles libpng, and avoids having hacky lines in our Makefile
(the compiler specification *should* be orthogonal to the build target!)

Also move the MinGW package install to the `install_deps.sh` script,
to move logic off of the YAML.

CMakeLists.txt

@@ -202,4 +202,5 @@ set(CPACK_STRIP_FILES ON) # Only applies to binary packages, not sources.
 set(CPACK_VERBATIM_VARIABLES ON)
 set(CPACK_THREADS 0) # Use all available CPU cores.
 set(CPACK_INCLUDE_TOPLEVEL_DIRECTORY OFF)
+
 include(CPack)