gallium: Fix install-gallium-links.mk on non-bash /bin/sh
gallium: Fix install-gallium-links.mk on non-bash /bin/sh

Debian uses dash by default, which doesn't do '+='.  Fixes servo's
osmesa-based headless testing system, which was looking for libOSMesa in
the lib/ directory.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Cc: 59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler