Update script find-xxdate to support opam

docs/manual/find-xxdate

@@ -9,7 +9,7 @@
 # Otherwise, a warning is printed on stderr and the script returns 1
 
 file=xxdate.exe
-directories="/usr/lib64/hevea /usr/share/hevea /usr/local/lib/hevea /opt/local/share/hevea"
+directories="$(opam var hevea:lib) /usr/lib64/hevea /usr/share/hevea /usr/local/lib/hevea /opt/local/share/hevea"
 
 for dir in ${directories}; do
   fullpath=${dir}/${file};