Do not include sys/ptrace.h and sys/user.h unconditionally.
Other portability fixes.

diff --git a/configure.in b/configure.in
index 68e1a32..61c2a67 100644
--- a/configure.in
+++ b/configure.in
@@ -792,6 +792,7 @@
 	sys/mount.h \
 	sys/msg.h \
 	sys/param.h \
+	sys/ptrace.h \
 	sys/reg.h \
 	sys/signal.h \
 	sys/shm.h \
@@ -800,6 +801,7 @@
 	sys/statfs.h \
 	sys/strtio.h \
 	sys/syscall.h \
+	sys/user.h \
 	sys/wait.h \
 	sys/v86.h \
 	sys/v86intr.h \