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

diff --git a/server/context_i386.c b/server/context_i386.c
index 9a48bcb..40fc894 100644
--- a/server/context_i386.c
+++ b/server/context_i386.c
@@ -14,14 +14,17 @@
 #include <sys/reg.h>
 #endif
 #include <unistd.h>
-#include <sys/ptrace.h>
+#ifdef HAVE_SYS_PTRACE_H
+# include <sys/ptrace.h>
+#endif
 #ifdef HAVE_SYS_PARAM_H
 # include <sys/param.h>
 #endif
-#include <sys/user.h>
+#ifdef HAVE_SYS_USER_H
+# include <sys/user.h>
+#endif
 
 #include "winbase.h"
-
 #include "thread.h"
 #include "request.h"