Add PYTHON_INSTALL_EXTRA_OPTIONS and --install-layout=deb

Debian packages should be installed under dist-packages, not
site-packages. Debian has patched distutils and setuptools to add a new
flag '--install-layout'. For --with-ipaplatform=debian,
PYTHON_INSTALL_EXTRA_OPTIONS is set to '--install-layout=deb'.

https://pagure.io/freeipa/issue/6764

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Timo Aaltonen <tjaalton@debian.org>
Reviewed-By: Martin Basti <mbasti@redhat.com>

Makefile.python.am

@@ -28,7 +28,8 @@ install-exec-local: $(top_builddir)/ipasetup.py
 		        --prefix "$(DESTDIR)$(prefix)" \
 		        --single-version-externally-managed \
 		        --record "$(DESTDIR)$(pkgpythondir)/install_files.txt" \
-		        --optimize 1; \
+		        --optimize 1 \
+		        $(PYTHON_INSTALL_EXTRA_OPTIONS); \
 	fi
 
 uninstall-local: