configure: enable the OpenCL ICD by default

configure: enable the OpenCL ICD by default

Nearly all the distributions* that build Mesa OpenCL, enable the ICD.
Since building a non-ICD driver has the chance of conflicting with
existing OpenCL binary (libOpenCL.so).

Furthermore, some applications expect the library to provide
annotated/versioned symbols.

https://lists.freedesktop.org/archives/mesa-dev/2017-September/171093.html

*Fedora, Suse, Arch, Debian, Ubuntu, FreeBSD use the ICD
Gentoo manages the conflicting files via eselect.

Cc: Matt Turner <mattst88@gmail.com>
Cc: Jan Vesely <jan.vesely@rutgers.edu>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-By: Aaron Watry <awatry@gmail.com>