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>

Makefile.am

@@ -35,6 +35,7 @@ AM_DISTCHECK_CONFIGURE_FLAGS = \
 	--enable-glx-tls \
 	--enable-nine \
 	--enable-opencl \
+	--enable-opencl-icd \
 	--enable-opengl \
 	--enable-va \
 	--enable-vdpau \

configure.ac

@@ -1255,9 +1255,9 @@ AC_ARG_ENABLE([opencl],
 AC_ARG_ENABLE([opencl_icd],
    [AS_HELP_STRING([--enable-opencl-icd],
           [Build an OpenCL ICD library to be loaded by an ICD implementation
-           @<:@default=disabled@:>@])],
+           @<:@default=enabled@:>@])],
     [enable_opencl_icd="$enableval"],
-    [enable_opencl_icd=no])
+    [enable_opencl_icd=yes])
 
 AC_ARG_ENABLE([gallium-tests],
     [AS_HELP_STRING([--enable-gallium-tests],