don't use which command in conf-povray

which seems to not be available on windows, so let's switch to use the --version command line option

Author: fccm2

packages/conf-povray/conf-povray.1/opam

@@ -14,6 +14,6 @@ depexts: [
   ["povray"] {os-family = "ubuntu"}
 ]
 build: [
-  ["sh" "-c" "which povray"]
+  ["sh" "-c" "povray --version"]
 ]
 flags: conf