Set "use system" variable if it has NOT already been explicitly set

By default, assuming all dependent projects of a project A are expected
to be available on the system if A is expected to be on the system. This
commit allow to overwrite this.

ExternalProjectDependency.cmake

@@ -545,7 +545,9 @@ macro(ExternalProject_Include_Dependencies project_name)
   set(_sb_REQUIRED_DEPENDS)
   foreach(dep ${_sb_DEPENDS})
     if(NOT ${_sb_proj} STREQUAL ${SUPERBUILD_TOPLEVEL_PROJECT})
-      if(_sb_USE_SYSTEM)
+      # Set "use system" variable if it has NOT already been explicitly set
+      get_property(_sb_${dep}_USE_SYSTEM_VAR GLOBAL PROPERTY SB_${dep}_USE_SYSTEM_VAR)
+      if(_sb_USE_SYSTEM AND NOT DEFINED ${_sb_${dep}_USE_SYSTEM_VAR})
         set_property(GLOBAL PROPERTY SB_${dep}_USE_SYSTEM ${_sb_USE_SYSTEM})
         #message(${_sb_proj} "Property SB_${dep}_USE_SYSTEM set to [${_sb_USE_SYSTEM_VAR}:${_sb_USE_SYSTEM}]")
       endif()