Add test to verify headers are self-contained

For each header, a translation unit is generated along with
corresponding compile target.
Currently, only boost/gil/*.hpp and boost/gil/io/* headres are included.

On CI services, compile self_contained_headers targets only if
TEST_HEADERS environment variable is set. This is to avoid build
timeouts due to CI services limits.
When running b2 locally, the tests are compiled by default.

test/self_contained_header.cpp

@@ -0,0 +1,18 @@
+//
+// Copyright (c) 2007-2015 Andrey Semashev
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+// This file contains a test boilerplate for checking that every public header
+// is self-contained and does not have any missing #include-s.
+
+#define BOOST_GIL_TEST_INCLUDE_HEADER() <boost/gil/BOOST_GIL_TEST_HEADER>
+
+#include BOOST_GIL_TEST_INCLUDE_HEADER()
+
+int main()
+{
+    return 0;
+}