Avoid to call unexisting variables (LD_LIBRARY_PATH, ...)

When we concatenate the variables PATH, R_INCLUDE, LD_LIBRARY_PATH, ... , some of them are not existing yet. Just test their existence with that *trick*: ${INCLUDE_PATH:+:$INCLUDE_PATH}".
Picked from [there](http://stackoverflow.com/questions/9631228/how-to-smart-append-ld-library-path-in-shell-when-nounset)