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>
diff --git a/Makefile.am b/Makefile.am
index 97cd43b..5ac0c9d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -6,8 +6,7 @@
 man_MANS = strace.1
 bin_SCRIPTS = strace-graph
 
-# OS is `linux'
-OS		= @opsys@
+OS		= linux
 # ARCH is `i386', `m68k', `sparc', etc.
 ARCH		= @arch@
 
@@ -28,8 +27,8 @@
 	     debian/strace.docs debian/strace.examples debian/strace.install \
 	     debian/strace.manpages debian/strace-udeb.install \
 	     strace.spec \
-	     strace-graph ChangeLog ChangeLog-CVS COPYRIGHT CREDITS PORTING \
-	     README-linux \
+	     strace-graph ChangeLog ChangeLog-CVS COPYRIGHT CREDITS \
+	     README-linux README-linux-ptrace \
 	     linux/ioctlsort.c linux/ioctlent.sh \
 	     linux/ioctlent.h.in linux/errnoent.h linux/signalent.h \
 	     linux/syscall.h linux/dummy.h \