Add stdbool.h for old MSVC.

src/libsodium/Makefile.am

@@ -228,3 +228,6 @@ endif
 
 SUBDIRS = \
 	include
+
+EXTRA_DIST = \
+	quirks/windows/stdbool.h

src/libsodium/quirks/windows/stdbool.h

@@ -0,0 +1,18 @@
+
+#ifndef __STDBOOL_H__
+#define __STDBOOL_H__
+
+#if defined(__cplusplus) || !defined(__STDC_VERSION__) || __STDC_VERSION__ < 199901L
+typedef signed char _Bool;
+# undef  bool
+# define bool _Bool
+
+# ifndef __bool_true_false_are_defined
+#  undef false
+#  define false 0
+#  undef true
+#  define true 1
+# endif
+#endif
+
+#endif