python3Packages.cffi: fix tests on FreeBSD

- disable a test, see comments.
- modify the existing patch to do what it claims to do (fixing
  compilation on clang) instead of only applying to darwin.

pkgs/development/python-modules/cffi/clang-pointer-substraction-warning.diff

@@ -1,11 +1,12 @@
-diff -r c649a735cf82 testing/support.py
---- a/testing/support.py	Thu Feb 23 05:42:01 2023 +0100
-+++ b/testing/support.py	Sat May 20 21:46:56 2023 -0400
-@@ -112,6 +112,7 @@
-                               '-Wno-unreachable-code']
-         # special things for clang
-         extra_compile_args.append('-Qunused-arguments')
-+        extra_compile_args.append('-Wno-null-pointer-subtraction')
-     else:
-         # assume a standard gcc
+diff --git a/testing/support.py b/testing/support.py
+index 063e52cf..ef0abdc7 100644
+--- a/testing/support.py
++++ b/testing/support.py
+@@ -117,6 +117,7 @@ else:
          extra_compile_args = ['-Werror', '-Wall', '-Wextra', '-Wconversion',
+                               '-Wno-unused-parameter',
+                               '-Wno-unreachable-code']
++extra_compile_args.append('-Wno-null-pointer-subtraction')
+
+ is_musl = False
+ if sys.platform == 'linux':

pkgs/development/python-modules/cffi/default.nix

@@ -72,6 +72,16 @@ else
         --replace '/usr/include/libffi' '${lib.getDev libffi}/include'
     '';
 
+    # it's kind of a miracle that this is all it takes for tests to work on FreeBSD.
+    # There are various comments in the source to the effect of assuming that anything that's
+    # not darwin or win32 must be linux and act exactly like linux
+    disabledTests = lib.optionals stdenv.isFreeBSD [
+      # this test assumes that libdl.dlopen actually works, but it does not on FreeBSD. You need
+      # to link against libc for the working function, but the function ends up being provided by
+      # the dynamic loader and so the test is not trivially fixable.
+      "test_dlopen_handle"
+    ];
+
     nativeBuildInputs = [
       pkg-config
       setuptools