MENU: add CMake file.
MENU: fix prototypes and unused variables.
MENU: fix signed/unsigned mismatches.
MENU: fix casts.
MENU: make assignments in conditionals more explicit.
MENU: fix intermixing & and |, leading to test always being false.
MENU: fix inappropriate reference.
MENU: suppress uninitialised 'selid' warning.

selid will be set by do_pullkeys or do_pullmouse whenever they return ret > 0.
MENU: suppress uninitialised 'ccx' and 'ccy' warning.

ccx and ccy will be initialised whenever cchild is not NULL.