This CL updates the build scripts to only install to glslang/SPIRV
instead of also installing to the SPIRV/ folder. The deprecation notice
is also removed.

Note, this may cause downstream build issues if include directories have
not been updated

Fixes #1964 #2216

Includes an additional deprecation and rationalizing the beginning sections.

This should enable glslang to be used as a dependency of other projects
that target WebAssembly, instead of assuming "EMSCRIPTEN" => "Want
glslang.js".

Knowing this in advance would have saved me some time.

This Cl updates the README.md to reference the deprecation date for the
current SPIRV/ install location.

Issue #1964

Allows building non-min-size builds with Emscripten.
Adds ENABLE_GLSLANG_WEB_DEVEL.
Moves the glslang.js files to OSDependent/Web.
Small cleanups and docs update.

You may need to install python 3.x and also
[windows users] may need to get it in your path in front of python 2.7.x.

If you don't need the external projects (i.e., SPIRV-Tools) and don't want
to upgrade to python 3, don't create the External subdirectory and CMake
won't require python 3.  (CMake has a bug that makes it find the wrong one
on a first pass if the top level does not require the version needed by
lower levels.)

Add to README: git config --global core.fileMode false

- Mention that the instructions are written for Bash.
- Quote the CMAKE_INSTALL_PREFIX to handle spaces in $(pwd).
- Remove SSH clone instructions. Instructions on how to clone are for
  those unfamiliar with GitHub, so fewer options are better and SSH
  requires additional configuration to make work.
- Replace ninja with make, because more people are familiar with it and
  we probably don't need to explain how to install it. Fixes #956.
- Remove the section mentioning cygwin, as it's unclear and doesn't
  seem necessary.
- Mention minimum language version in the Dependencies section.
- Move `cd $BUILD_DIR` out of the Linux configuration section because
  it's needed on both platforms. Add a line about creating the build
  directory, too.

Also added known-good mechanism to fetch latest validated spirv-tools.
Also added -Od and -Os to disable optimizer and optimize for size.

Fetching spirv-tools is optional for both glsl and hlsl. Legalization
of hlsl is done by default if spirv-opt is present at cmake time.
Optimization for glsl is currently done through the option -Os.

Legalization testing is currently only done on four existing shaders.
A separate baseLegalResults directory holds those results. All previous
testing is done with the optimizer disabled.

Added an alternative link for cloning the repo in the build instructions