package/apr-util: fix build with per-package directories

With per-package directories support enabled, the build of apr-util
fails, for two reasons:

 - The rules.mk file is generated by the 'apr' package, and then
   copied into the 'apr-util' source directory. This is done by the
   'apr-util' build process. Unfortunately, this rules.mk file has a
   number of hardcoded paths: to the compiler and to the libtool
   script.

   Due to this, the compiler from the 'apr' per-package directory gets
   used. But this compiler uses the 'apr' package sysroot, which does
   not have all the dependencies of the 'apr-util' package, causing
   the build to fail because <expat.h> is not found.

 - Similarly, the libtool script itself has some hardcoded paths,
   which make it use the compiler/linker from the 'apr' per-package
   directory, so it does not find the expat library.

We fix both issues by doing the necessary replacement in both rules.mk
and libtool.

Fixes:

  http://autobuild.buildroot.net/results/2a67b5d58f79348e20a972125e4797eff5585716/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>