Copy the const.py file created in build to your source directory if you want to use your source directory to work with Gramps:
cp build/lib.linux-x86_64-2.7/gramps/gen/const.py gramps/gen/const.py
# note the lib.linux path may vary depending on your system e.g. it could be lib.linux-i686-2.7
python Gramps.py