Switch macOS static build to CMake (#1938)

This moves the tortuous flags out of our CI scripts,
and allows us to benefit from the libpng download caching.

cmake/macos-static.cmake

@@ -0,0 +1,28 @@
+# This file is meant to be included at the project level,
+# in order to generate executables compatible with old macOS versions.
+# See our `macos-static` CMake preset for how it's meant to be used.
+
+# The `-mmacosx-version-min=10.9` flag ensures that the binary only uses APIs available on Mac OS X 10.9 Mavericks.
+# The `-arch` flags build a "fat binary" that works on both Apple architectures:
+# older Intel x64 Macs and newer ARM "Apple Silicon" ones.
+set("-mmacosx-version-min=10.9 -arch x86_64 -arch arm64")
+set(CMAKE_C_FLAGS "${secret_sauce}" CACHE STRING "Flags used by the C compiler during all build types.")
+set(CMAKE_CXX_FLAGS "${secret_sauce}" CACHE STRING "Flags used by the CXX compiler during all build types.")
+
+# Mac OS X has always provided zlib, so we can safely link dynamically against it.
+# However, libpng is *not* provided by default, so we link it statically, which requires downloading and building it from source.
+set(PNG_SHARED OFF)
+set(PNG_STATIC ON)
+# If libpng is already available (e.g. via Homebrew), we ignore that and still build our own.
+set(FETCHCONTENT_TRY_FIND_PACKAGE_MODE NEVER)
+# But we still want to attempt linking against the system's zlib.
+function(rgbds_provide_dependency method dep_name)
+  if(dep_name STREQUAL "ZLIB")
+    find_package(ZLIB)
+    if(ZLIB_FOUND)
+      FetchContent_SetPopulated(ZLIB)
+    endif()
+  endif()
+endfunction(rgbds_provide_dependency)
+cmake_language(SET_DEPENDENCY_PROVIDER rgbds_provide_dependency
+               SUPPORTED_METHODS FETCHCONTENT_MAKEAVAILABLE_SERIAL)