meson: allow building GLES1, GLES2, OpenGL independently of each other
$ size build-*/src/gallium/targets/dri/libgallium_dri.so
text data bss dec hex filename
24051730 391608 212648 24655986 1783872 build-gles1-no-gles2-no-gl/src/gallium/targets/dri/libgallium_dri.so
24087566 391608 213672 24692846 178c86e build-gles2-no-gles1-no-gl/src/gallium/targets/dri/libgallium_dri.so
24735918 391608 212488 25340014 182a86e build-gles-and-gl/src/gallium/targets/dri/libgallium_dri.so
24090454 391608 214888 24696950 178d876 build-gles-no-gl/src/gallium/targets/dri/libgallium_dri.so
24687762 391608 211496 25290866 181e872 build-gl-no-gles/src/gallium/targets/dri/libgallium_dri.so
2.6% smaller between gl+gles1+gles2 and gles1+gles2 (without gl) (iris only, debug build)