Only use glsl-unit if it is available

Previously a fresh checkout of ol3 would try to use glsl-unit to minify the WebGL shaders. This was because a git checkout gives an identical timestamp to all checked out files, and so the build system would see that the generated JavaScript (`*shader.js`) was not newer than its dependency (`*.glsl`) and so would attempt to rebuild the generated JavaScript.

This patch fixes this by only adding the rule to build the JavaScript if glsl-unit is installed.