configure: Shell cleanup

- Remote unnecessary quote characters
- Change some uses of “test "$foo"” to “test -n "$foo"”
- Merge “if/else/if” into “if/elif”
- Wrap lines longer than 80 characters and add a test to “make check”

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>