Trac #30757: fedora-33-standard/homebrew-macos flint/arb breakage

`fedora-33-standard`, https://github.com/sagemath/sage/runs/1235092189

1. `configure` finds system `flint` but not system `arb` (headers are
installed by the fedora package in `/usr/include/arb/`, but the arb
spkg-configure expects them in `/usr/include`).

2. Then compiling arb spkg results in:
{{{
  [arb-2.16.0.p0]
/sage/local/var/tmp/sage/build/arb-2.16.0.p0/src/fmpr.h:25:10: fatal
error: flint/config.h: No such file or directory
  [arb-2.16.0.p0]      25 | #include "flint/config.h"
  [arb-2.16.0.p0]         |          ^~~~~~~~~~~~~~~~
  [arb-2.16.0.p0]   compilation terminated.
}}}
Indeed, `/usr/include/flint/` is present, but `config.h` is missing.

For Sage 9.3, we can do the upgrade to arb 2.18.1 (#28623) and flint
2.6.3 (#29719). But we need a quick fix for Sage 9.2.

(`fedora-33` is expected to be released 2020-10-20.)

Same error also on `homebrew-macos` with flint installed.

URL: https://trac.sagemath.org/30757
Reported by: mkoeppe
Ticket author(s): Dima Pasechnik
Reviewer(s): Matthias Koeppe

Author: Release Manager

build/pkgs/arb/patches/flint26comp.patch

@@ -0,0 +1,48 @@
+From d3d9983231e0f034e86a1e75761627eb8213b704 Mon Sep 17 00:00:00 2001
+From: fredrik <[email protected]>
+Date: Wed, 29 Apr 2020 13:41:36 +0200
+Subject: [PATCH] handle flint incompatibilities
+
+---
+ fmpr.h        | 4 ++++
+ fmpz_extras.h | 4 ++++
+ 2 files changed, 8 insertions(+)
+
+diff --git a/fmpr.h b/fmpr.h
+index 2204d72e..c6a8c342 100644
+--- a/fmpr.h
++++ b/fmpr.h
+@@ -22,7 +22,11 @@
+ #include "flint/flint.h"
+ #include "flint/fmpz.h"
+ #include "flint/fmpq.h"
++#if __FLINT_RELEASE < 20600
+ #include "flint/config.h"
++#else
++#include "flint/flint-config.h"
++#endif
+ #include "fmpz_extras.h"
+ 
+ #ifndef flint_abort
+diff --git a/fmpz_extras.h b/fmpz_extras.h
+index 4fd0538f..486e28c8 100644
+--- a/fmpz_extras.h
++++ b/fmpz_extras.h
+@@ -43,6 +43,8 @@ fmpz_add_inline(fmpz_t z, const fmpz_t x, const fmpz_t y)
+         fmpz_add(z, x, y);
+ }
+ 
++#if __FLINT_RELEASE < 20600
++
+ static __inline__ void
+ fmpz_add_si(fmpz_t z, const fmpz_t x, slong y)
+ {
+@@ -61,6 +63,8 @@ fmpz_sub_si(fmpz_t z, const fmpz_t x, slong y)
+         fmpz_add_ui(z, x, -y);
+ }
+ 
++#endif
++
+ static __inline__ void
+ fmpz_add_si_inline(fmpz_t z, const fmpz_t x, slong y)
+ {