Add option ENABLE_CTEST to skip testing

Update spirv-tools known good.

Make  buildbot licenses consistent with the rest of the code

Web: Extend with compute stage, SSBOs, and atomics on SSBO memory.

The Fuchsia platform build and the Chromium one use a completely
different set of configs to specify default warnings. The previous
version of BUILD.gn worked with Chromium, but failed to build
with Fuchsia.

This CL fixes the issue by identifying said configs and reusing
them consistently.

Change-Id: I7de526a57d2f14eb93c03e06401d2c3059d35e9a

Looks like will add about 1K compressed size to executable footprint.

Add Bazel build files and bots for them

Only apply volatile semantics to atomics when using Vulkan Memory Model

In order to avoid potential build failures that could arise from
SPIRV-Tools/Headers, and to avoid reading known_good.json, this build
configuration does not build the SPIRV-Tools dependency and hence cannot
perform HLSL compilation to SPIR-V.

Make runtests configurable.

Fix config issues and revert previous fixes for semi-colon issues

Added an option to make pch enabled in Cmake

This CL allows setting the location of glslangValidator and spirv-remap
when calling the runtests script. A test target is added to CMake to
execute runtests and sets the location to the build folder for the two
applications.

This reverts commit 41261d95.
This reverts commit 7c9accb6.

Instead of trying to work around all of the potential semicolon issues
in glslang, making it conform to Chromium's style, mark the code
explicitly as non-chromium in the BUILD.gn, so chromium doesn't
attempt to enforce its style rules on glslang.

Fixes #1931

Export glslang targets on installation

find Python and add External subdir only if BUILD_EXTERNAL option was set

find Python and add External subdir only if BUILD_EXTERNAL option was set (on by default) and /External dir exists