readline: prohibit parallel building

we run in the problem desriped at [1], which seems to
be a race condition. What happens is that in the install
phase libreadline.so.6.3 is rebuild, but _not_ linked
against ncurses

[1] http://thread.gmane.org/gmane.comp.gnu.readline/485

readline/Pkgfile

@@ -13,8 +13,8 @@ build() {
     cd $name-${version:0:3}
 
     ./configure --prefix=/usr --mandir=/usr/man
-    make SHLIB_LIBS=-lncurses
-    make DESTDIR=$PKG install
+    make -j1 SHLIB_LIBS=-lncurses
+    make -j1 DESTDIR=$PKG install
 
     install -d $PKG/lib
     mv $PKG/usr/lib/lib*.so.* $PKG/lib