Basic changes for GNU/Hurd (#676)

* Define OSMINOR & OS_HURD on GNU/Hurd

Add a way to identify GNU/Hurd with b2, and also in the Python support.

* Use /proc/self/exe for executable_path on Hurd

Use the Linux compatibility procfs translator to get the full path of
the current executable.

* Define _GNU_SOURCE on any GNU libc-based OS

Make sure to enable GNU features when building on any OS that uses
GNU libc.

src/engine/jam.cpp

@@ -747,7 +747,7 @@ char * executable_path( char const * argv0 )
     sysctl( mib, 4, buf, &size, NULL, 0 );
     return ( !size || size == sizeof( buf ) ) ? NULL : strndup( buf, size );
 }
-#elif defined(__linux__) || defined(__CYGWIN__)
+#elif defined(__linux__) || defined(__CYGWIN__) || defined(__GNU__)
 # include <unistd.h>
 char * executable_path( char const * argv0 )
 {