Avoid adding all kinds of things to the toolchain's flags that within
Gentoo (Prefix) we really shouldn't, such as sysroot, deployment target,
arch, etc.

--- a/tools/darwin.jam
+++ b/tools/darwin.jam
@@ -240,6 +240,9 @@
         }
     }
 
+    # leave compiler flags etc. up to the toolchain
+    return $(version-feature) ;
+
     if $(version-feature)
     {
         if $(.debug-configuration)