Fix error when cross-compiling for macOS arm64 using Makefile

Fix "Architecture arm64 is not supported when cross compiling" and
also be more be more flexible for manually specifying CC or the
osxcross darwin version (which corresponds with a macOS SDK version
to build against).

Makefile

@@ -467,6 +467,11 @@ ifeq ($(PLATFORM),darwin)
   BASE_CFLAGS += -mmacosx-version-min=$(MACOSX_VERSION_MIN) \
                  -DMAC_OS_X_VERSION_MIN_REQUIRED=$(MAC_OS_X_VERSION_MIN_REQUIRED)
 
+  MACOSX_ARCH=$(ARCH)
+  ifeq ($(ARCH),x86)
+    MACOSX_ARCH=i386
+  endif
+
   ifeq ($(ARCH),ppc)
     BASE_CFLAGS += -arch ppc
     ALTIVEC_CFLAGS = -faltivec
@@ -498,15 +503,31 @@ ifeq ($(PLATFORM),darwin)
   endif
 
   ifeq ($(CROSS_COMPILING),1)
-    ifeq ($(ARCH),x86_64)
-      CC=x86_64-apple-darwin13-cc
-      RANLIB=x86_64-apple-darwin13-ranlib
-    else
-      ifeq ($(ARCH),x86)
-        CC=i386-apple-darwin13-cc
-        RANLIB=i386-apple-darwin13-ranlib
-      else
-        $(error Architecture $(ARCH) is not supported when cross compiling)
+    # If CC is already set to something generic, we probably want to use
+    # something more specific
+    ifneq ($(findstring $(strip $(CC)),cc gcc),)
+      CC=
+    endif
+
+    ifndef CC
+      ifndef DARWIN
+        # macOS 10.9 SDK
+        DARWIN=13
+        ifneq ($(findstring $(ARCH),ppc ppc64),)
+          # macOS 10.5 SDK, though as of writing osxcross doesn't support ppc/ppc64
+          DARWIN=9
+        endif
+        ifeq ($(ARCH),arm64)
+          # macOS 11.3 SDK
+          DARWIN=20.4
+        endif
+      endif
+
+      CC=$(MACOSX_ARCH)-apple-darwin$(DARWIN)-cc
+      RANLIB=$(MACOSX_ARCH)-apple-darwin$(DARWIN)-ranlib
+
+      ifeq ($(call bin_path, $(CC)),)
+        $(error Unable to find osxcross $(CC))
       endif
     endif
   endif