#include <stdio.h>
#include <string.h>

...

typedef struct {

    bool show;

    xyz_double position;
    GLfloat ambient[4];
    GLfloat diffuse[4];
    GLfloat specular[4];

} Light;

// Standard colours
static const GLfloat white[4] = {1, 1, 1, 1};
static const GLfloat grey[4] = {0.5, 0.5, 0.5, 1};
static const GLfloat black[4] = {0, 0, 0, 1};

// Default light colours

Light defaultlight;
memcpy(defaultlight.ambient, grey, sizeof(grey));

-----

compiler:
(line with memcpy): error: expected ‘)’ before ‘.’ token