Trac #30745: Allow using arb, flint and ntl from Homebrew

- ntl/flint/singular: https://groups.google.com/d/msg/sage-
devel/KXK_zxzfhIQ/0MaSLhvMAAAJ
- ecl: Reported in https://groups.google.com/d/msg/sage-
devel/gFaUNw4v3aw/P-dTJ5IOCgAJ

`/usr/local` is leaking into our build, through wrong orders of include
and/or library directives.

Critical for 9.3 because it affects a major supported platform. We
should at least give a configure-time error if these installations are
found on homebrew if we cannot fix this.

URL: https://trac.sagemath.org/30745
Reported by: mkoeppe
Ticket author(s): Dima Pasechnik
Reviewer(s): John Palmieri

build/pkgs/arb/distros/homebrew.txt

@@ -1,3 +1 @@
-## This package depends on ntl, the homebrew package of which we cannot use
-## because it is built with NTL_THREADS.  See https://trac.sagemath.org/ticket/29339
-# sagemath/science/arb
+arb

build/pkgs/flint/distros/homebrew.txt

@@ -1,3 +1 @@
-## This package depends on ntl, the homebrew package of which we cannot use
-## because it is built with NTL_THREADS.  See https://trac.sagemath.org/ticket/29339
-# sagemath/science/flint
+flint

build/pkgs/ntl/distros/homebrew.txt

@@ -1,3 +1 @@
-## We cannot use ntl on homebrew because it is built with NTL_THREADS.
-## See https://trac.sagemath.org/ticket/29339
-# ntl
+ntl

src/sage/rings/polynomial/plural.pyx

@@ -367,6 +367,8 @@ cdef class NCPolynomialRing_plural(Ring):
             True
             sage: H is loads(dumps(H))  # indirect doctest
             True
+            sage: A2.<x,y,z> = FreeAlgebra(GF(5), 3)
+            sage: R2 = A2.g_algebra({y*x:x*y-z, z*x:x*z+2*x, z*y:y*z-2*y}, order=TermOrder('degrevlex', 2))
 
         Check that :trac:`17224` is fixed::