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This uses the new configure option --with-default-config=PATH.

The foreign option is set in AM_INIT_AUTOMAKE already

Basic kate support is possible without needing libkate

This makes it easy to know which things will be enabled or
disabled, depending which deps were found or not found.

This rewritten configure.ac is nearly independent of any of the xiph m4
macros, most of which are ancient and can nowadays easily replaced
with pkg-config.