Add possibility for dependency on string configuration tags

Currently it is possible to make tags depend on other "bool"  tags (e.g. PLANTUML_JAR depends on HAVE_DOT).
There is no way to make tags depend on empty "string" tags. This patch fixes this limitation.

This dependency can be used for the newly proposed PLANTUML_INCLUDE_PATH tag (pull request 226, Support plantuml !include statement ) which in makes only sense in case PLANTUML_JAR is set.