Ensure -I and -L arguments exists before using

When generating -I and -L flags the path variables used as arguments can be
empty, resulting in a command line where -I and -L eats the next command line
argument which can be just anything.
Fix broken post processing of delayed include paths

To automatically compute the include paths for internal libraries a full parse
of all makerules.mk files must be complete so that include-directories are
found. Include directives to these directories must then be added as a post
processing step to the artifacts that linked with a particular library.

A previous change erroneously used the default <target>_INCL variable for this
purpose, the only problem is that that variable is expanded early in the
process, at the setup of each target. Adding additional content to <target>_INCL
therefore has no effect. A separate post processing include variable must
therefore exist for each target.