Do not pass -DNDEBUG to Python compilations in development mode

Do not pass -DNDEBUG to Python compilations in development mode

The Python CFLAGS include -DNDEBUG.  This was apparently done
intentionally -- setting the flags is done manually because, according
to a comment, python-config passes too many things to the compiler
(which is true).

Per PR python/20445, this patch changes configure so that -DNDEBUG is
only used by release builds.  This probably doesn't have very much
effect in practice, but I did see that some Python headers use assert,
so perhaps it will give some safety.

Tested by rebuilding and re-running gdb.python/*.exp on x86-64 Fedora 28.

gdb/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	PR python/20445:
	* configure: Rebuild.
	* configure.ac: Conditionally use -DNDEBUG for Python.