Added a check that the platform API macros are in sync with Boost.System.

Check that BOOST_FILESYSTEM_POSIX/WINDOWS_API and BOOST_POSIX/WINDOWS_API
are defined in the same way and error out if not. Allow users to suppress
this check (and keep their code compiling) by defining
BOOST_FILESYSTEM_ALLOW_SYSTEM_API_MISMATCH to 1 to reduce the error to
a warning or to 2 to completely disable the check.

If we diverge in platform API selection with Boost.System again, we may
want to implement our own system category that is guaranteed to be in sync
with the platform API Boost.Filesystem actually uses. But for now the two
libraries are expected to be in sync, and the added check should do.

Author: Lastique

doc/release_history.html

@@ -42,7 +42,7 @@
 
 <h2>1.91.0</h2>
 <ul>
-  <li>Boost.Filesystem now defines and uses its own platform macros <code>BOOST_FILESYSTEM_POSIX_API</code> and <code>BOOST_FILESYSTEM_WINDOWS_API</code>. The old <code>BOOST_POSIX_API</code> and <code>BOOST_WINDOWS_API</code> macros are still defined by Boost.System.</li>
+  <li>Boost.Filesystem now defines and uses its own platform macros <code>BOOST_FILESYSTEM_POSIX_API</code> and <code>BOOST_FILESYSTEM_WINDOWS_API</code>. The old <code>BOOST_POSIX_API</code> and <code>BOOST_WINDOWS_API</code> macros are still defined by Boost.System. Boost.Filesystem will check that the two sets of macros are in sync and by default will fail to compile if not. The compilation error can be reduced to a warning by defininig <code>BOOST_FILESYSTEM_ALLOW_SYSTEM_API_MISMATCH</code> to 1 or disabled entirely by defining it to 2. Note that in this case the error code values reported by Boost.Filesystem will not match the error category.</li>
   <li>On POSIX platforms not supporting <code>openat</code> and related APIs, fixed an error reported by <code>recursive_directory_iterator</code> increment when the iterator encounters a dangling symlink and following symlinks is disabled.</li>
   <li><b>Breaking change for Cygwin users.</b> The library now treats Cygwin as a POSIX platform. This has several user-facing consequences, in particular:
     <ul>

include/boost/filesystem/config.hpp

@@ -66,6 +66,18 @@
 #define BOOST_FILESYSTEM_POSIX_API
 #endif
 
+#if !defined(BOOST_FILESYSTEM_ALLOW_SYSTEM_API_MISMATCH) || (BOOST_FILESYSTEM_ALLOW_SYSTEM_API_MISMATCH < 2)
+#include <boost/system/api_config.hpp>
+#if (defined(BOOST_FILESYSTEM_POSIX_API) && !defined(BOOST_POSIX_API)) || (defined(BOOST_FILESYSTEM_WINDOWS_API) && !defined(BOOST_WINDOWS_API))
+#if (BOOST_FILESYSTEM_ALLOW_SYSTEM_API_MISMATCH < 1)
+#error Boost.Filesystem: Platform API mismatch between Boost.System and Boost.Filesystem, reported error code values will not match the error category
+#else
+#include <boost/config/pragma_message.hpp>
+BOOST_PRAGMA_MESSAGE("Boost.Filesystem: Platform API mismatch between Boost.System and Boost.Filesystem, reported error code values will not match the error category")
+#endif
+#endif
+#endif
+
 //  throw an exception  ----------------------------------------------------------------//
 //
 //  Exceptions were originally thrown via boost::throw_exception().