use GNUMake set-if-unset construct (?=) for build variables

[benkirk]

use GNUMake/BSDMake set-if-unset construct (?=) for build variables, so we can use a common Make_defs and override select settings.

This allows, for example,

export MURaM_HOME_DIR="$(pwd)"
export CUDA_HOME=""
export FFT_MODE="FFTW"
export FFT_MODE="HEFFTE_CPU"
export FFTW3_HOME=/container/fftw/3.3.10
export HEFFTE_HOME=/container/heffte/github-main
export OPT="-O2"
export DBG=""

export CC="$(which mpicc)"
export CCC="$(which mpicxx) -std=c++11"
export LD="${CCC}"

make

without requirng a specific Make_defs instance.