Use the same system for INCLUDES and LIBRARIES directories

[dolanor]

I have a dependency on a boost library which is not installed in the default system directories but in a subdirectory of the project.
When I set
TGT_LDFLAGS := -L../../3rdParty/boost/lib in a directory, and that I try to compile from the parent directory, it doesn't find the library because the command line tries to find the lib 1 level too high.
That system should work relatively like the Includes dir with -I