Eliminate use of FLINT's NTL interface

[mkoeppe]

We currently insist that the FLINT library is built with its NTL interface.

This interface consists of just a few functions that could easily be provided instead by a small library separate from FLINT, or included in sagelib. https://github.com/wbhart/flint2/blob/trunk/interfaces/NTL-interface.cpp

This could help with modularization. Also the use of NTL by Singular is minimal.

flintlib/flint#1039

CC: @kiwifb @antonio-rojas

Component: refactoring

Issue created by migration from https://trac.sagemath.org/ticket/32908

[mkoeppe]

Description changed:

--- 
+++ 
@@ -3,3 +3,6 @@
 This interface consists of just a few functions that could easily be provided instead by a small library separate from FLINT, or included in sagelib.  https://github.com/wbhart/flint2/blob/trunk/interfaces/NTL-interface.cpp
 
 This could help with modularization. Also the use of NTL by Singular is minimal.
+
+https://github.com/wbhart/flint2/issues/1039
+

[mezzarobba]

Maybe no longer relevant now the NTL interface is header-only?

[mkoeppe]

I agree