Expose ROOTLIBDIR to cling::DynamicLibraryManager: (#4710)

DynamicLibraryManager only looks at LD_LIBRARY_PATH and friends, but for install / prefix
builds that might not point to the ROOT library dir. Make it explicit that the
DynamicLibraryManager is supposed to look at the directory containing ROOT libraries.
Fixes install-with-prefix builds with RPATH, that now do not need a fake LD_LIBRARY_PATH
anymore.

include/cling/Interpreter/DynamicLibraryManager.h

@@ -95,6 +95,10 @@ namespace cling {
        return m_SearchPaths;
     }
 
+    void addSearchPath(llvm::StringRef dir) {
+      m_SearchPaths.emplace_back(SearchPathInfo{dir, /*IsUser*/ true});
+    }
+
     ///\brief Looks up a library taking into account the current include paths
     /// and the system include paths.
     ///\param[in] libStem - The filename being looked up