use the host python when running the h2py code, and have that search sysroot

--- a/Tools/scripts/h2py.py
+++ b/Tools/scripts/h2py.py
@@ -60,6 +60,7 @@ except KeyError:
             pass
 
 def main():
+    sysroot = os.getenv('SYSROOT', '')
     global filedict
     opts, args = getopt.getopt(sys.argv[1:], 'i:')
     for o, a in opts:
@@ -72,7 +73,7 @@ def main():
             process(sys.stdin.buffer, sys.stdout.buffer)
         else:
             filename = filename.encode()
-            fp = open(filename, 'rb')
+            fp = open(sysroot + filename, 'rb')
             outfile = os.path.basename(filename)
             i = outfile.rfind(b'.')
             if i > 0: outfile = outfile[:i]