Cannot compile C++11 code via Makefile
As I turns out, the avr-gcc (GCC) 4.9.1 goodies weren't being used at all! The arduino package was using a decrepit version of gcc,

prakhar@sim74stic ~ $ /usr/share/arduino/hardware/tools/avr/bin/avr-g++ --version
avr-g++ (GCC) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

prakhar@sim74stic ~ $ /usr/share/arduino/hardware/tools/avr/bin/avr-gcc --version
avr-gcc (GCC) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


All I had to do was specify:

AVR_TOOLS_DIR = /usr


to exexute with -std=c++11 flag (I also had to add an -fpermissive flag to compile C with g++).