build: Makefile: include config.mk

So that CC and friends are set correctly.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
tests: framework: Use $(CC) instead of gcc

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
tests: core: GetPutPixel: Fix warnings.

Fix "This decimal constant is unsigned only in ISO C90" warnings.

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