Improve /bin/sh compliance.

	* build-all.sh: Separate out export of PATH environment variable
	for /bin/sh compliance.
	* check-results.sh: Use /bin/sh, not /bin/bash, use = not == in
	tests.
	* run-tests.sh: Separate out export of DEJAGNU, AVRTEST_HOME and
	PATH environment variables for /bin/sh compliance.