Add autoconf magic to support Bitrig

Bitrig is an openbsd fork.  Patch from dhill.  Ticket 6982.

changes/6982

@@ -0,0 +1,3 @@
+  o Minor features (portability):
+    - Tor now builds correctly on Bitrig, an OpenBSD fork. Patch from dhill.
+      Ticket 6982.

configure.ac

@@ -743,7 +743,7 @@ if test x$transparent = xtrue ; then
    if test x$transparent_ok = x1 ; then
      AC_DEFINE(USE_TRANSPARENT, 1, "Define to enable transparent proxy support")
      case $host in
-       *-*-openbsd*)
+       *-*-openbsd* | *-*-bitrig*)
          AC_DEFINE(OPENBSD, 1, "Define to handle pf on OpenBSD properly") ;;
      esac
    else
@@ -1252,7 +1252,7 @@ if test x$enable_gcc_warnings = xyes || test x$enable_gcc_warnings_advisory = xy
   CFLAGS="$save_CFLAGS"
 
   case $host in
-    *-*-openbsd*)
+    *-*-openbsd* | *-*-bitrig*)
       # Some OpenBSD versions (like 4.8) have -Wsystem-headers by default.
       # That's fine, except that the headers don't pass -Wredundant-decls.
       # Therefore, let's disable -Wsystem-headers when we're building