Always set macos_version with a reasonable default.

bmk_config.bmx

@@ -65,7 +65,7 @@ Global opt_gprof:Int
 Global opt_dumpbuild
 
 'Global cfg_platform$
-Global macos_version
+Global macos_version:Int=2784 ' 10.14
 
 Global app_main$
 Global app_type$

bmk_ng.bmx

@@ -28,9 +28,7 @@ LoadBMK(AppDir + "/make.bmk", True)
 LoadBMK(AppDir + "/config.bmk")
 
 ' add some defaults
-If processor.Platform() = "macos"
-	globals.SetVar("macos_version", String(macos_version))
-End If
+globals.SetVar("macos_version", String(macos_version))
 globals.SetVar("cc_opts", New TOptionVariable)
 globals.SetVar("ld_opts", New TOptionVariable)
 globals.SetVar("c_opts", New TOptionVariable)