ifdef mode

M-x hide-ifdef-mode             Load mode
M-x hide-ifdefs (C-c @ h)       Explicitly hide ifdefs
M-x show-ifdefs (C-c @ s)       Undo hide-ifdefs
M-x hide-ifdef-define (C-c @ d) To define a symbol.
M-x hide-ifdef-undef (C-c @ u)  To undefine a symbol.