Fix binary stdin/stdout io in Windows

Make --font-file accept "-" to mean stdin, and have it work
in Windows too!

configure.ac

diff --git a/configure.ac b/configure.ac
index 8245181..e1b6bf8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -53,7 +53,7 @@
 
 # Functions and headers
 AC_CHECK_FUNCS(mprotect sysconf getpagesize mmap)
-AC_CHECK_HEADERS(unistd.h sys/mman.h)
+AC_CHECK_HEADERS(unistd.h sys/mman.h io.h)
 
 # Compiler flags
 AC_CANONICAL_HOST