CONFIG: ptscotch header not found on ArchLinux (fixes #1877)

- on ArchLinux, everything is installed under /usr/include/scotch.

  The detection script uses SCOTCH_ARCH_PATH as an initial guess for
  ptscotch as well. However, on the second pass, it has an absolute
  value ("/usr") instead of a logical one ("scotch-system").
  This resulted in the logic for handling scotch+ptscotch subdirs
  being bypassed.

wmake/scripts/have_scotch

@@ -90,6 +90,7 @@ search_scotch()
     then
         header=$(findFirstFile \
             "$prefix/include/$localDir/$incName" \
+            "$prefix/include/scotch/$incName" \
             "$prefix/include/$incName" \
         )
         library=$(findExtLib "$libName")
@@ -201,12 +202,15 @@ search_ptscotch()
         header=$(findFirstFile  \
             "$prefix/include/$FOAM_MPI/$incName" \
             "$prefix/include/$localDir/$incName" \
+            "$prefix/include/ptscotch/$incName" \
+            "$prefix/include/scotch/$incName" \
             "$prefix/include/$incName"
         )
         library="$(findExtLib $FOAM_MPI/$libName $libName)"
     elif isSystem "$prefix"
     then
         header=$(findFirstFile \
+            "/usr/local/include/$localDir/$incName" \
             "/usr/local/include/ptscotch/$incName" \
             "/usr/local/include/scotch/$incName" \
             "/usr/local/include/$incName" \