meson: subdir() into glsl and mesa only when building OpenGL drivers

I found out about these while building only one Vulkan driver, and flex or bison weren’t installed. These two programs are required to build the GLSL or ARB shader parsers, but neither are useful on Vulkan which uses SPIR-V.

This makes a meson build go from ~2.5s to ~2.25s on my computer, but this shouldn’t matter much. It also prevents libmesa_sse41.a from being needlessly built.

This patch is more easily read with git log -p -w to ignore indent changes.

Signed-off-by: Emmanuel Gil Peyrot linkmauve@linkmauve.fr