Describe the bug

As discussed in the ROOT forum:
WIthout the memstat feature, root.exe still offers the -memstat option but produced failures like

input_line_11:2:6: error: unknown type name 'TMemStat'
 new TMemStat("",100000,5000000);

Expected behavior

If memstat is not part of the build, the memstat option should not be offered or there should be a clearer error message.

To Reproduce

Steps to reproduce the behavior:

1. Compile with cmake -Dmemstat=off
2. Run root -l

Setup

1. Master
2. Arch Linux
3. Built from sources