Define SRCDIR more correctly in configure.ac

Without this fix, we were just getting SRCDIR=`pwd`, which naturally
was breaking out-of-tree builds.

Bugfix on becae4c943969a4f4f14423cc897d39f41af7773; bug not in any
released Tor.

configure.ac

@@ -1879,7 +1879,7 @@ AH_TEMPLATE([BUILDDIR],[tor's build directory])
 AC_DEFINE_UNQUOTED(BUILDDIR,"$BUILDDIR")
 
 if test "x$SRCDIR" = "x"; then
-  SRCDIR=`pwd`
+  SRCDIR=$(cd "$srcdir"; pwd)
 fi
 AH_TEMPLATE([SRCDIR],[tor's sourcedir directory])
 AC_DEFINE_UNQUOTED(SRCDIR,"$SRCDIR")