Only output -L -rpath cruft if using a non-standard install location.

Makes ABI / cross-compiling easier on the soul.

--- a/sdl-config.in
+++ b/sdl-config.in
@@ -44,7 +44,12 @@
       echo -I@includedir@/SDL @SDL_CFLAGS@
       ;;
 @ENABLE_SHARED_TRUE@    --libs)
-@ENABLE_SHARED_TRUE@      echo -L@libdir@ @SDL_RLD_FLAGS@ @SDL_LIBS@
+@ENABLE_SHARED_TRUE@      if test x"${prefix}" != x"/usr" ; then
+@ENABLE_SHARED_TRUE@          libdirs="-L@libdir@"
+@ENABLE_SHARED_TRUE@      else
+@ENABLE_SHARED_TRUE@          libdirs=""
+@ENABLE_SHARED_TRUE@      fi
+@ENABLE_SHARED_TRUE@      echo $libdirs @SDL_RLD_FLAGS@ @SDL_LIBS@
 @ENABLE_SHARED_TRUE@      ;;
 @ENABLE_STATIC_TRUE@@ENABLE_SHARED_TRUE@    --static-libs)
 @ENABLE_STATIC_TRUE@@ENABLE_SHARED_FALSE@    --libs|--static-libs)