Switch from AC_CANONICAL_SYSTEM to AC_CANONICAL_BUILD and AC_CANONICAL_HOST.

The autoconf manual suggests that AC_CANONICAL_SYSTEM should not be
used, instead the _HOST, _BUILD, _TARGET versions should be used as
required.

ChangeLog:

	* configure.ac: Switch from AC_CANONICAL_SYSTEM to
	AC_CANONICAL_BUILD and AC_CANONICAL_HOST.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* doc/Makefile.in: Regenerate.
	* src/*/Makefile.in: Regenerate.