Make lest_expect_abort more portable to non-Windows platforms

Unconditionally include `cstdio` for `fflush` and friends, and `cstdlib`
for `abort`. When `_WIN32` is not defined, include `unistd.h` instead of
`io.h` for `dup` and `dup2`.

contrib/lest_expect_abort/Readme.md

@@ -15,7 +15,7 @@ Please note the following:
 
 Dependencies
 ------------
-*lest_expect_abort* requires headers `io.h` and `fcntl.h` for suppression of messages from `assert`.
+*lest_expect_abort* requires header `fcntl.h` and `io.h` (Windows) or `unistd.h` (otherwise) for suppression of messages from `assert`.
 
 
 Assertion macros

contrib/lest_expect_abort/lest_expect_abort.hpp

@@ -9,8 +9,14 @@
 #include "lest/lest.hpp"
 
 #include <csetjmp>
+#include <cstdio>
+#include <cstdlib>
 
+#if _WIN32
 #include <io.h>
+#else
+#include <unistd.h>
+#endif
 #include <fcntl.h>
 
 #if ! defined( lest_NO_SHORT_MACRO_NAMES )

contrib/lest_expect_abort/lest_expect_abort_cpp03.hpp

@@ -9,8 +9,14 @@
 #include "lest/lest_cpp03.hpp"
 
 #include <csetjmp>
+#include <cstdio>
+#include <cstdlib>
 
+#if _WIN32
 #include <io.h>
+#else
+#include <unistd.h>
+#endif
 #include <fcntl.h>
 
 #if ! defined( lest_NO_SHORT_MACRO_NAMES )