Only dump -L/-rpath if libdir is non-standard

--- a/smpeg-config.in
+++ b/smpeg-config.in
@@ -45,11 +45,13 @@
       echo $includes -I@includedir@/smpeg `@SDL_CONFIG@ --cflags`
       ;;
     --libs)
+      if [ "@libdir@" != "/usr/lib" ]; then
       if [ "`uname`" = "SunOS" ]; then
         libdirs="-L@libdir@ -R@libdir@"
       else
         libdirs="-L@libdir@ @SMPEG_RLD_FLAGS@"
       fi
+      fi
       echo $libdirs -lsmpeg `@SDL_CONFIG@ --libs`
       ;;
     *)