Fixed the confusing and inconsistently used names *_LINK_FLAGS and *_LINKER_FLAGS by always setting and getting both variables

- Quotes around input variables when doing string replace avoid existing semicolons from being deleted.
- The project target library itself was missing in the linker flags.

fixed issue with compiler/linker flags being interpreted as lists and thus having semicolons in between

${PROJECT_NAME}_CPPFLAGS -> ${PROJECT_NAME}_CXX_FLAGS
${PROJECT_NAME}_LDFLAGS -> ${PROJECT_NAME}_LINK_FLAGS

also make sure that the exported lists are consistently space-separated (sometimes they might be semicolon separated or mixed)