targets/opencl: don't hardcode the icd file install to /etc/...

Use $(sysconfdir) instead of hardcoding /etc.

While the OpenCL spec expects the file in /etc, people building their
stack can override that, esp. !Linux users.

Furthermore this removes a fundamental violation, which results in the
system file being overwritten even as one explicitly sets --prefix
and/or DESTDIR.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-By: Aaron Watry <awatry@gmail.com>
(cherry picked from commit 0cd09585)