[bin/oil] Fix the interpreter reported by --version.

This broke when we removed _OVM_IS_BUNDLE from the environment.  Instead
we now check for the absence of sys.executable.

Python-2.7.13/Python/sysmodule.c

@@ -1448,12 +1448,7 @@ _PySys_Init(void)
     SET_SYS_FROM_STRING("platform",
                         PyString_FromString(Py_GetPlatform()));
     /* getpath.c was removed */
-#ifdef OVM_MAIN
-    /* 'import platform' uses sys.executable for a default arg to libc_ver().
-     * Stub it out with a dummy. */
-    SET_SYS_FROM_STRING("executable",
-                        PyString_FromString("OVM_DUMMY_STRING"));
-#else
+#ifndef OVM_MAIN
     SET_SYS_FROM_STRING("executable",
                         PyString_FromString(Py_GetProgramFullPath()));
 #endif

core/util.py

@@ -271,8 +271,8 @@ def ShowAppVersion(app_name):
   assert py_compiler.endswith(']'), py_compiler
   py_compiler = py_compiler[1:-1]
 
-  # This environment variable set in C code.
-  py_impl = 'OVM' if posix.environ.get('_OVM_IS_BUNDLE') else 'CPython'
+  # We removed sys.executable from sysmodule.c.
+  py_impl = 'CPython' if hasattr(sys, 'executable') else 'OVM'
 
   # What C functions do these come from?
   print('%s version %s' % (app_name, version))