Add WASM build target for Web version of glslang

This adds build rules to support generating a WASM binary to be used
on the web. The API exposed to web applications is definated in the
new glslang.js.cpp file.

glslang/CMakeLists.txt

@@ -122,3 +122,15 @@ if(ENABLE_GLSLANG_INSTALL)
         install(FILES ${file} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/glslang/${dir})
     endforeach()
 endif(ENABLE_GLSLANG_INSTALL)
+
+if(ENABLE_GLSLANG_WEB)
+    add_executable(glslang.js glslang.js.cpp)
+    glslang_set_link_args(glslang.js)
+    target_link_libraries(glslang.js glslang SPIRV)
+    if(EMSCRIPTEN)
+        set_target_properties(glslang.js PROPERTIES
+          OUTPUT_NAME "glslang"
+          SUFFIX ".js"
+          LINK_FLAGS "--bind")
+    endif(EMSCRIPTEN)
+endif(ENABLE_GLSLANG_WEB)