fix cmakefile (part of an upstream commit) to avoid packaging failure if

openimageio (specifically iv) isn't already installed. reported by aja@

graphics/openimageio/patches/patch-src_doc_CMakeLists_txt

@@ -0,0 +1,24 @@
+$OpenBSD: patch-src_doc_CMakeLists_txt,v 1.1 2020/11/19 12:07:59 sthen Exp $
+
+check whether we're going to build iv, don't check if it's already installed.
+part of
+
+From 33f19f3da1e5e01aa63c711d2fd3ee7ff9310248 Mon Sep 17 00:00:00 2001
+From: Richard <hobbes1069@gmail.com>
+Date: Mon, 5 Mar 2018 11:38:29 -0600
+Subject: [PATCH] Make building and installing of documentation more packaging
+ friendly. (#1890)
+
+Index: src/doc/CMakeLists.txt
+--- src/doc/CMakeLists.txt.orig
++++ src/doc/CMakeLists.txt
+@@ -19,8 +19,7 @@ if (UNIX AND TXT2MAN AND PYTHONINTERP_FOUND)
+     message (STATUS "Unix man page documentation will be generated")
+     set (cli_tools oiiotool iinfo maketx idiff igrep iconvert)
+ 
+-    find_program (IV_FOUND iv)
+-    if (IV_FOUND)
++    if (TARGET iv)
+         list (APPEND cli_tools iv)
+     endif()
+