Fix the build of the native gtk-update-icon-cache when cross-compiling.

Fix the build of the native gtk-update-icon-cache when cross-compiling.

As a noinst_PROGRAMS, the libtool generated for cross-compiling will be
used, which will mess up the linking. Create a all-local target instead.
Also ensure that building uses always a native version of the tool by
specifying a GTK_UPDATE_ICON_CACHE automake variable.
Finally "config.h" has been created to work for the target platform and
causes problem when cross-compiling. So we temporarily generate a basic
config.h which contains only the strict minimum.