Update m4 files and Makefiles.
* acinclude.m4: Use URL and add SPDX identifier. * m4/noexecstack.m4: Likewise. * Makefile.am: Likewise. * doc/Makefile.am: Likewise. * mpi/Makefile.am: Likewise. * tests/Makefile.am: Likewise.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>