Renamed Make variable DIME_PROJECT_ROOT to topdir
Renamed Make variable DIME_PROJECT_ROOT to topdir

The variable name was so dominant and long that the other variables
were not easy to read.

lavabit/libdime,lavabit/libdime,greyg00s/libdime,greyg00s/libdime,lavabit/libdime,lavabit/libdime