release 1.7.11.rc2
extract the program configuration functionality into a separate tools.rb file
set a LIB_DIR constant for the benefit of autoproj

Autoproj needs the content of tools.rb for its bootstrap file, so provide
a LIB_DIR constant to give it the full path to it.
release 1.7.11.rc3
env: avoid generating path variables with empty entries (::) when inheriting