[PATCH] Use kde4-config for determining KDE's library and include directories.

[[[
At the moment the kde4-config script is used only to determine KDE4's
prefix, and then its header and library locations are derived manually.

This is not optimal, as it assumes the libraries are always installed
into $kde_prefix/lib{64} and the headers in $kde_prefix/include. The
latter is especially problematic as the headers are often installed in a
subdirectory of include/ so they do not conflict with their KDE
Frameworks 5 counterparts.

* build/ac-macros/kwallet.m4: Use the `--install include' and `--install
  lib' kde4-config parameters to retrieve the location where KDE4's
  headers and libraries are installed, respectively.
]]]