build: Change library names GP -> gfxprim

Don't forget to rerun configure

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
demos: c_simple: Small cleanup.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
GFXprim 1.0.0-rc1

Signed-off-by: Cyril Hrubis <metan@ucw.cz>