Etc.h

float RadToDegree(float rad)
{
	return (360.0 * rad) / (2.0 * M_PI);     
}

float DegreeToRad(float degree)
{
	return (degree * 2.0 * M_PI) / 360.0;
}

void InverseModelView(GLfloat *_m)
{
	GLfloat m[16];
	glGetFloatv(GL_MODELVIEW_MATRIX, m);
	for(int i = 0; i < 16; i++) _m[i] = 0;
	/*
	a[0] a[4] a[8] a[12]
	a[1] a[5] a[9] a[13]
	a[2] a[6] a[10]a[14]
	a[3] a[7] a[11]a[15]
	*/
	/*
	_m[0] = m[0];
	_m[5] = m[5];
	_m[10] = m[10];
	_m[1] = m[4];
	_m[4] = m[1];
	_m[2] = m[8];
	_m[8] = m[2];
	_m[6] = m[9];
	_m[9] = m[6];
	_m[15] = 1.0;
	*/
	_m[0] = m[0];
	_m[5] = m[5];
	_m[10] = m[10];
	_m[1] = m[1];
	_m[4] = m[4];
	_m[2] = m[2];
	_m[8] = m[8];
	_m[6] = m[6];
	_m[9] = m[9];
	_m[15] = 1.0;
}

Sky.h

GLuint skyTexture;
GLUquadric *skyQuadric;
const float skyRadius = 1000.0;

void InitSky()
{
	BITMAP *bmp = load_bitmap("sky.bmp", 0);
	skyTexture = allegro_gl_make_texture(bmp);
	destroy_bitmap(bmp);
	skyQuadric = gluNewQuadric();
	gluQuadricTexture(skyQuadric, true);
}

void RenderSky()
{
	glCullFace(GL_FRONT);
	//glDisable(GL_DEPTH_TEST);
	glDepthMask(false);
	glPushMatrix();
	GLfloat m[16];
	InverseModelView(m);
	glLoadMatrixf(m);
	glRotatef(90, 1.0, 0.0, 0.0);
	glBindTexture(GL_TEXTURE_2D, skyTexture);
	gluSphere(skyQuadric, skyRadius, 10, 20);
	glPopMatrix();
	glDepthMask(true);
	//glEnable(GL_DEPTH_TEST);
	glCullFace(GL_BACK);
}