groff: improve reproducibility

groff: improve reproducibility

As said in ${S}/m4/groff.m4
...
1642 # gdiffmk will attempt to use bash (for option -ef of 'test'). If bash
1643 # is not available it will use /bin/sh.
...

So drop hardcode shebang replacement, and pass variable to configure,
it also remove build path in gdiffmk to improve reproducibility

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>