Include math.h. Add extern declaration for atof in case

Include math.h.  Add extern declaration for atof in case
we don't see one.
(Qscreen_gamma): New.
(struct x_frame_parm_table): Add prototypes, add
x_set_screen_gamma.
(gamma_correct): New function.
(defined_color): Call it.
(x_set_screen_gamma): New.
(x_set_title): Add parameter old_value.
(RES_TYPE_FLOAT): New.
(x_get_arg): Handle RES_TYPE_FLOAT.
(Fx_create_frame): Call x_default_parameter for `screen-gamma'.
(lookup_pixel_color): Change call to x_alloc_nearest_color to
new prototype.
(lookup_rgb_color): Ditto.
(syms_of_xfns): Initialize Qscreen_gamma.