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>

compat/plan9/main.c

@@ -0,0 +1,34 @@
+/*
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+int plan9_main(int argc, char **argv);
+
+#undef main
+int main(int argc, char **argv)
+{
+    /* The setfcr() function in lib9 is broken, must use asm. */
+#ifdef __i386
+    short fcr;
+    __asm__ volatile ("fstcw        %0 \n"
+                      "or      $63, %0 \n"
+                      "fldcw        %0 \n"
+                      : "=m"(fcr));
+#endif
+
+    return plan9_main(argc, argv);
+}