allow setting compiler flags so that a custom installation location for libtiff can be supported

[yxqd]

This is an attempt to allow users to set compiler flags when running setup.py, so that a custom installation location for libtiff can be handled. This can be useful, for example, when installing srtm4 under a conda environment (where libtiff was installed using conda). For example, after this change, I was able to run python setup.py build in a conda environment by

export LDLIBS="-L $CONDA_PREFIX/lib"
export CFLAGS="-I $CONDA_PREFIX/include"
python setup.py build

[yxqd]

This is replaced by #8