BUILD: make 51D easier to build by defaulting to 51DEGREES_SRC

Till now 3 paths were needed, 51DEGREES_SRC, 51DEGREES_INC, and
51DEGREES_LIB.  Let's make the last two default to 51DEGREES_SRC since
it's the same location, and fix the doc to reflect this (all three were
documented but inconsistently).
2 files changed