Remove parts of automake machinery which are not needed on Linux

This change is a verbatim part of Dmitry's changes to remove support
for non-Linux architectures.

* Makefile.am: Don't install PORTING file. Install README-linux-ptrace file.
Set OS variable to linux unconditionally.
* configure.ac: Remove code to set opsys variable, and its usage.
Remove checks for headers which are never present on Linux.
* m4/stat.m4: Remove 'ifdef LINUX' check.
* m4/statfs.m4: Likewise.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
4 files changed