[CMake] Add an include directory for installed ROOT targets.

When using an installed ROOT, the targets didn't have include
directories. Here, the default include directory for an installed ROOT
is added to all targets.

Author: hageboeck

cmake/modules/RootMacros.cmake

@@ -1346,6 +1346,11 @@ function(ROOT_STANDARD_LIBRARY_PACKAGE libname)
                        )
   endif(ARG_OBJECT_LIBRARY)
 
+  # Include directories relative to the installed location for installed targets:
+  if (PROJECT_NAME STREQUAL ROOT)
+    target_include_directories(${libname} PUBLIC $<INSTALL_INTERFACE:include>)
+  endif()
+
   if (NOT (ARG_HEADERS OR ARG_NODEPHEADERS))
     message(AUTHOR_WARNING "Called with no HEADERS and no NODEPHEADER. The generated "
       "dictionary will be empty. Consider using ROOT_LINKER_LIBRARY instead.")