Issue #1040439: better document how to compile and link an embedded Python interpreter.

Still lacks docs for Windows (anyone?).

Doc/extending/embedding.rst

@@ -133,8 +133,9 @@ The code to run a function defined in a Python script is:
 
 This code loads a Python script using ``argv[1]``, and calls the function named
 in ``argv[2]``.  Its integer arguments are the other values of the ``argv``
-array.  If you compile and link this program (let's call the finished executable
-:program:`call`), and use it to execute a Python script, such as::
+array.  If you :ref:`compile and link <compiling>` this program (let's call
+the finished executable :program:`call`), and use it to execute a Python
+script, such as::
 
    def multiply(a,b):
        print("Will compute", a, "times", b)
@@ -257,37 +258,52 @@ write the main program in C++, and use the C++ compiler to compile and link your
 program.  There is no need to recompile Python itself using C++.
 
 
-.. _link-reqs:
+.. _compiling:
 
-Linking Requirements
-====================
+Compiling and Linking under Unix-like systems
+=============================================
 
-While the :program:`configure` script shipped with the Python sources will
-correctly build Python to export the symbols needed by dynamically linked
-extensions, this is not automatically inherited by applications which embed the
-Python library statically, at least on Unix.  This is an issue when the
-application is linked to the static runtime library (:file:`libpython.a`) and
-needs to load dynamic extensions (implemented as :file:`.so` files).
+It is not necessarily trivial to find the right flags to pass to your
+compiler (and linker) in order to embed the Python interpreter into your
+application, particularly because Python needs to load library modules
+implemented as C dynamic extensions (:file:`.so` files) linked against
+it.
 
-The problem is that some entry points are defined by the Python runtime solely
-for extension modules to use.  If the embedding application does not use any of
-these entry points, some linkers will not include those entries in the symbol
-table of the finished executable.  Some additional options are needed to inform
-the linker not to remove these symbols.
+To find out the required compiler and linker flags, you can execute the
+:file:`python{X.Y}-config` script which is generated as part of the
+installation process (a generic :file:`python3-config` script is also
+available).  This script has several options, of which the following will
+be directly useful to you:
 
-Determining the right options to use for any given platform can be quite
-difficult, but fortunately the Python configuration already has those values.
-To retrieve them from an installed Python interpreter, start an interactive
-interpreter and have a short session like this::
+* ``pythonX.Y-config --cflags`` will give you the recommended flags when
+  compiling::
 
-   >>> import distutils.sysconfig
-   >>> distutils.sysconfig.get_config_var('LINKFORSHARED')
+   $ /opt/bin/python3.2-config --cflags
+   -I/opt/include/python3.2m -I/opt/include/python3.2m -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes
+
+* ``pythonX.Y-config --ldflags`` will give you the recommended flags when
+  linking::
+
+   $ /opt/bin/python3.2-config --ldflags
+   -I/opt/lib/python3.2/config-3.2m -lpthread -ldl -lutil -lm -lpython3.2m -Xlinker -export-dynamic
+
+.. note::
+   To avoid confusion between several Python installations (and especially
+   between the system Python and your own compiled Python), it is recommended
+   that you use the absolute path to :file:`python{X.Y}-config`, as in the above
+   example.
+
+If this procedure doesn't work for you (it is not guaranteed to work for
+all Unix-like platforms; however, we welcome bug reports at
+http://bugs.python.org),  you will have to read your system's documentation
+about dynamic linking and/or examine Python's Makefile and compilation
+options.  In this case, the :mod:`sysconfig` module is a useful tool to
+programmatically extract the configuration values that you will want to
+combine together::
+
+   >>> import sysconfig
+   >>> sysconfig.get_config_var('LINKFORSHARED')
    '-Xlinker -export-dynamic'
 
-.. index:: module: distutils.sysconfig
-
-The contents of the string presented will be the options that should be used.
-If the string is empty, there's no need to add any additional options.  The
-:const:`LINKFORSHARED` definition corresponds to the variable of the same name
-in Python's top-level :file:`Makefile`.
 
+.. XXX similar documentation for Windows missing