platform.hpp: Indent so I can see wtf is going on with the preprocessor control flow.

Look for ``__linux__`` on linux instead of ``linux`` because compiling with
-std=c++11 removes the ``linux`` preprocessor symbol.

vm/platform.hpp

@@ -1,7 +1,6 @@
 #if defined(WINDOWS)
   #if defined(WINNT)
     #include "os-windows.hpp"
-
     #if defined(FACTOR_AMD64)
       #include "os-windows.64.hpp"
     #elif defined(FACTOR_X86)
@@ -28,8 +27,7 @@
     #endif
   #else
     #include "os-genunix.hpp"
-
-#if defined(linux)
+    #if defined(__linux__)
       #define FACTOR_OS_STRING "linux"
       #include "os-linux.hpp"