build: Plan 9 support

This adds support for building on Plan 9 x86-32.  The compat/plan9
directory contains these items:

- replacements for the 'head' and 'printf' shell commands
- wrapper for main() to disable FPU exceptions

Larger required changes to the system are described in the
documentation.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/os_support.h

@@ -58,7 +58,7 @@ static inline int is_dos_path(const char *path)
     return 0;
 }
 
-#if defined(__OS2__)
+#if defined(__OS2__) || defined(__Plan9__)
 #define SHUT_RD 0
 #define SHUT_WR 1
 #define SHUT_RDWR 2