You don't need the Arduino IDE, you can simply use a Makefile and write your
C code as long as you have avr-gcc, avr-libc and avrdude installed!

make            # compile and generate main.hex 
make flash      # flash it to the Arduino nano

CAUTION: This makefile is for getting started with bare/from-scratch
development, you still need to link extra stuff yourself and take into
account header files if you want to make use of some of Arduino's
libs and whatnot. If you want a Makefile that comes with batteries included
check the arduino-make folder.