[Cmake] Prevent Headercheck from polluting the Cache

Cmake by default added a dependeny variable for each headercheck
target. Given the amount of such targets and the fact, that these
dependencies should not be changed by the user, this is unwanted
behaviour. The patch fixes this by removing the variable from the
cache.

cmake/modules/Headercheck.cmake

@@ -67,5 +67,6 @@ macro(finalize_headercheck)
     set_property(TARGET headercheck_${targname} APPEND_STRING PROPERTY COMPILE_FLAGS "-DHEADERCHECK -I${PROJECT_SOURCE_DIR}/${relpath} -I${CMAKE_BINARY_DIR}")
     set_property(TARGET headercheck_${targname} PROPERTY ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/headercheck/${relpath}")
     add_dune_all_flags(headercheck_${targname})
+    unset(headercheck_${targname}_LIB_DEPENDS CACHE)
   endforeach(header ${headerlist})
 endmacro(finalize_headercheck)