Makefile: directory creation fix

"make install" command fails with error
"mkdir: cannot create directory
‘/usr/lib/gcc/x86_64-redhat-linux/4.8.1/plugin/python’:
File exists"

The patch adds "-p" option to mkdir invocation.
Work-in-progress support for gcc 4.9

This patch is a work-in-progress that makes the plugin closer to compilable
against what will become gcc 4.9 (currently gcc trunk; specifically r205593
patched to install various missing headers).

This is mostly:
  (A) tracking down where declarations have landed in the new layout
  (B) adding new headers to the list of those to be installed
  (C) some porting due to "real" changes e.g. passes becoming C++ classes.
  (D) adding -fno-rtti when building against 4.9 or later