Split "is emscripten" config from "enable glslang.js"

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

README.md

@@ -182,7 +182,8 @@ Use the steps in [Build Steps](#build-steps), with the following notes/exception
   + execute `updateGrammar web` from the glslang subdirectory
     (or if using your own scripts, `m4` needs a `-DGLSLANG_WEB` argument)
   + set `-DENABLE_HLSL=OFF -DBUILD_TESTING=OFF -DENABLE_OPT=OFF -DINSTALL_GTEST=OFF`
-  + turn on `-DENABLE_GLSLANG_WEB=ON`
+  + turn on `-DENABLE_GLSLANG_JS=ON`
+  + optionally, for a minimum-size binary, turn on `-DENABLE_GLSLANG_WEBMIN=ON`
   + optionally, for GLSL compilation error messages, turn on `-DENABLE_GLSLANG_WEB_DEVEL=ON`
 * `emsdk` needs to be present in your executable search path, *PATH* for
   Bash-like environments