Issue 1453
extras/pch.py: Experimental extension to enable precompiled headers for clang++ and g++

Signed-off-by: Thomas Nagy <tnagy2pow10@gmail.com>
moved pch out of demos (requires extras/pch.py) and disabled the dependency on boost
missing file

Signed-off-by: Thomas Nagy <tnagy2pow10@gmail.com>
Issue 1454