gallium: correctly manage libsensors link flags

We should be using LIBS rather than the LDFLAGS variable. Furthermore
try to keep the linking to the final stage, rather than intermetent
static library.

Cc: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
3 files changed