configure.in: Set and subsitute host_exeext.

	* configure.in: Set and subsitute host_exeext.  Use it when creating
	the assembler and linker symlinks.
	* configure: Rebuild.
	* Makefile.in (exeext): Set to @host_exeext@.
	(build_exeext): New variable, set to @build_exeext@.
	(FLAGS_TO_PASS): Pass down build_exeext.
	(STAGESTUFF): Use build_exeext, not exeext, for gen* and bi*
 	programs.

From-SVN: r17267