libtiff: Disable OpenGL entirely for Darwin

On the Hydra builders and with Clang 7, CMake fails to find OpenGL. With
Clang 11 it manages to find the the OpenGL and GLUT system frameworks
for me. It does not find `gl.h` however. So the tests for `tiffgt` fail
on a missing include.

Since previously OpenGL wasn't detected I've opted to disable the CMake
check for OpenGL for darwin. This means `tiffgt.c` is never built and
tested. Ideally we'd provide the proper headers so the tests can
succeed.

pkgs/development/libraries/libtiff/0001-Disable-OpenGL.patch

···
+From 6385dfa45c86b79da4f84df10f481f0ba260ab1f Mon Sep 17 00:00:00 2001
+From: toonn <[e-mail]>
+Date: Sat, 3 Jul 2021 20:38:25 +0200
+Subject: [PATCH] Disable OpenGL
+
+---
+ CMakeLists.txt | 3 ---
+ 1 file changed, 3 deletions(-)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index efe857d..e72897b 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -611,9 +611,6 @@ endif()
+ find_package(OpenGL)
+ find_package(GLUT)
+ set(HAVE_OPENGL FALSE)
+-if(OPENGL_FOUND AND OPENGL_GLU_FOUND AND GLUT_FOUND)
+-  set(HAVE_OPENGL TRUE)
+-endif()
+ # Purely to satisfy the generated headers:
+ check_include_file(GL/gl.h HAVE_GL_GL_H)
+ check_include_file(GL/glu.h HAVE_GL_GLU_H)
+-- 
+2.17.2 (Apple Git-113)
+

pkgs/development/libraries/libtiff/default.nix

···
   };
 
   # FreeImage needs this patch
-  patches = [ ./headers.patch ];
+  patches = [ ./headers.patch ]
+    # CMake manages to detect the OpenGL and GLUT system frameworks but doesn't
+    # find the `gl.h` header, making the tests fail. With older Clangs OpenGL
+    # wasn't detected so the simplest fix was to keep CMake from checking for
+    # it. This means `tiffgt` isn't built and tested.
+    ++ lib.optional stdenv.isDarwin ./0001-Disable-OpenGL.patch;
 
   outputs = [ "bin" "dev" "dev_private" "out" "man" "doc" ];