Eliminate 'default' MODEL, instead compute from uname -m

posix.mak

@@ -48,11 +48,21 @@ ifeq (,$(OS))
      $(error Unrecognized or unsupported OS for uname: $(uname_S))
 endif
 
-MODEL:=default
-ifneq (default,$(MODEL))
-      MODEL_FLAG:=-m$(MODEL)
+ifeq (,$(MODEL))
+    uname_M:=$(shell uname -m)
+    ifeq (x86_64,$(uname_M))
+        MODEL=64
+    else
+        ifeq (i686,$(uname_M))
+            MODEL=32
+        else
+            $(error Cannot figure 32/64 model from uname -m: $(uname_M))
+        endif
+    endif
 endif
 
+MODEL_FLAG:=-m$(MODEL)
+
 override PIC:=$(if $(PIC),-fPIC,)
 
 # Configurable stuff that's rarely edited