gnu-build-system: Fix "libdir" and "includedir" for multiple-output drvs.

* guix/build/gnu-build-system.scm (configure): Change `--libdir' and
  `--includedir' to LIBDIR + "/lib" and INCLUDEDIR + "/include",
  respectively.

guix/build/gnu-build-system.scm

@@ -64,10 +64,11 @@
                   "--enable-fast-install"
                   (string-append "--prefix=" prefix)
                   `(,@(if libdir
-                          (list (string-append "--libdir=" libdir))
+                          (list (string-append "--libdir=" libdir "/lib"))
                           '())
                     ,@(if includedir
-                          (list (string-append "--includedir=" includedir))
+                          (list (string-append "--includedir="
+                                               includedir "/include"))
                           '())
                     ,@configure-flags)))))