Make --with-simgrid-* take precedence over pkg-config

So one can specify a precise location and not get bothered by a
system-installed version.

configure.ac

@@ -152,14 +152,14 @@ if test x$enable_simgrid = xyes ; then
 	PKG_CHECK_MODULES([SIMGRID], [simgrid], [], [:])
 
 	if test "$simgrid_include_dir" != "no" ; then
-	   	SIMGRID_CFLAGS="$SIMGRID_CFLAGS -I$simgrid_include_dir"
+		SIMGRID_CFLAGS="-I$simgrid_include_dir $SIMGRID_CFLAGS"
 	fi
 	if test "$simgrid_lib_dir" != "no" ; then
-	   	SIMGRID_LIBS="$SIMGRID_LIBS -L$simgrid_lib_dir"
+		SIMGRID_LIBS="-L$simgrid_lib_dir $SIMGRID_LIBS"
 	fi
 	if test "$simgrid_dir" != "no" ; then
-	   	SIMGRID_CFLAGS="$SIMGRID_CFLAGS -I$simgrid_dir/include"
-	   	SIMGRID_LIBS="$SIMGRID_LIBS -L$simgrid_dir/lib"
+		SIMGRID_CFLAGS="-I$simgrid_dir/include $SIMGRID_CFLAGS"
+		SIMGRID_LIBS="-L$simgrid_dir/lib $SIMGRID_LIBS"
 	fi
    	if test -n "$SIMGRID_CFLAGS" ; then
 	   	CFLAGS="$SIMGRID_CFLAGS $CFLAGS"