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If a function returns Lisp_Object it must be declared, otherwise on a 64-bit
system, it will return just 32 bits (int).

* configure.in: Use -Wimplicit-function-declaration if compiler supports it.

* src/keyboard.h (poll_for_input_1): Unconditionally declare.

* src/xterm.h (x_get_focus_frame): Declare.