[Docs] Mention C/C++ system libraries on "Building Emscripten from Source" page (#22842)

Author: Antony74

site/source/docs/building_from_source/index.rst

@@ -13,6 +13,11 @@ before it can be used (e.g. ``npm install``).  The ``bootstrap`` script in the
 top level of the repository takes care of running these steps and ``emcc`` will
 error out if it detects that ``bootstrap`` needs to be run.
 
+Emscripten comes with its own versions some C/C++ system libaries which ``emcc``
+builds automatically as and when needed (in the emsdk builds, these are precompiled).
+You can also build them manually with the ``embuilder`` tool - see ``embuilder --help``
+for more information.
+
 In addition to the main emscripten repository you will also need to checkout
 and build LLVM and Binaryen (as detailed below).  After compiling these, you
 will need to edit your ``.emscripten`` file to point to their corresponding