Web: Selectively remove a few key features, using #ifndef GLSLANG_WEB

Save about 100K.

N.B.: This is done by eliminating a function call, at a high level,
not by #ifdef'ing a bunch of code.

Also, removed no longer needed *_EXTENSION #ifdef in the code not
needed by GLSLANG_WEB.

CMakeLists.txt

@@ -79,6 +79,10 @@ if(ENABLE_HLSL)
     add_definitions(-DENABLE_HLSL)
 endif(ENABLE_HLSL)
 
+if(ENABLE_GLSLANG_WEB)
+    add_definitions(-DGLSLANG_WEB)
+endif(ENABLE_GLSLANG_WEB)
+
 if(WIN32)
     set(CMAKE_DEBUG_POSTFIX "d")
     if(MSVC)