Update to sagemath 8.5.

Also:
- Bundle ipython again; Fedora version is too far ahead.  Also have to bundle
  prompt_toolkit since Fedora version is too far ahead of bundled ipython.
- Drop LANGUAGES variable setting, now ignored by the sagemath build system.
- Drop unused SAGE_CBLAS variable from /usr/bin/sage.
- Do not force the C locale when launching sagemath.
- Allow the user to override SAGE_DEBUG in /usr/bin/sage.
- Add -ecm, -giac, and -latte patches to fix interactions with external tools.
- Add -sigfpe patch from upstream.

sagemath-buildroot.patch

@@ -1,5 +1,6 @@
---- src/setup.py.orig	2018-10-16 09:07:30.086993782 -0600
-+++ src/setup.py	2018-10-16 09:10:20.586710425 -0600
+diff -up src/setup.py.orig src/setup.py
+--- src/setup.py.orig	2019-01-04 11:05:11.337077724 -0700
++++ src/setup.py	2019-01-16 09:45:32.570933224 -0700
 @@ -387,6 +387,9 @@ class sage_build_cython(Command):
          with open(self._version_file, 'w') as f:
              f.write(self._version_stamp)