bpo-36721: Add --embed option to python-config (GH-13500)

To embed Python into an application, a new --embed option must be
passed to "python3-config --libs --embed" to get "-lpython3.8" (link
the application to libpython). To support both 3.8 and older, try
"python3-config --libs --embed" first and fallback to "python3-config
--libs" (without --embed) if the previous command fails.

Add a pkg-config "python-3.8-embed" module to embed Python into an
application: "pkg-config python-3.8-embed --libs" includes
"-lpython3.8".  To support both 3.8 and older, try "pkg-config
python-X.Y-embed --libs" first and fallback to "pkg-config python-X.Y
--libs" (without --embed) if the previous command fails (replace
"X.Y" with the Python version).

On the other hand, "pkg-config python3.8 --libs" no longer contains
"-lpython3.8". C extensions must not be linked to libpython (except
on Android, case handled by the script); this change is backward
incompatible on purpose.

"make install" now also installs "python-3.8-embed.pc".
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index 68843e2..8c2b40d 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -149,6 +149,24 @@
 stable ABI.
 (Contributed by Victor Stinner in :issue:`36722`.)
 
+To embed Python into an application, a new ``--embed`` option must be passed to
+``python3-config --libs --embed`` to get ``-lpython3.8`` (link the application
+to libpython). To support both 3.8 and older, try ``python3-config --libs
+--embed`` first and fallback to ``python3-config --libs`` (without ``--embed``)
+if the previous command fails.
+
+Add a pkg-config ``python-3.8-embed`` module to embed Python into an
+application: ``pkg-config python-3.8-embed --libs`` includes ``-lpython3.8``.
+To support both 3.8 and older, try ``pkg-config python-X.Y-embed --libs`` first
+and fallback to ``pkg-config python-X.Y --libs`` (without ``--embed``) if the
+previous command fails (replace ``X.Y`` with the Python version).
+
+On the other hand, ``pkg-config python3.8 --libs`` no longer contains
+``-lpython3.8``. C extensions must not be linked to libpython (except on
+Android, case handled by the script); this change is backward incompatible on
+purpose.
+(Contributed by Victor Stinner in :issue:`36721`.)
+
 f-strings now support =  for quick and easy debugging
 -----------------------------------------------------