On Debian ${name}.pc gets install into

${libdir}/x86_64-linux-gnu/pkgconfig/.
Honor this setting when checking whether the module is installed.

Fixes flyspray 1100 for me.

[[Imported from SVN: r6746]]

lib/dunemodules.lib

@@ -69,7 +69,8 @@ parse_control() {
   # - source module: ${path}/dune.module
   #   and there is a file ${path}/${name}.pc.in
   local path="$(canonicalpath "$1")"
-  if test -f $path/../../pkgconfig/${name}.pc; then
+  if test -f $path/../../pkgconfig/${name}.pc -o\
+     -f $path/../../x86_64-linux-gnu/pkgconfig/${name}.pc; then
 	  path=$(canonicalname "$path/../../../")
       export module_inst="yes"
   fi