Fix incompatibilities with the musl C library (#156)

This makes ksh build on Alpine Linux which uses this C library.

src/lib/libast/include/ast_std.h:
- Define __DEFINED_FILE to hide FILE internals from the Korn
  shell's SFIO.

src/lib/libast/features/wchar:
- Include wchar.h before redefining iswalpha() to avoid mangling
  the C library's declaration.

src/lib/libast/features/lib:
- Test whether off64_t and off_t are actually distinct types before
  using the former.

Fixes: #3

src/lib/libast/include/ast_std.h

@@ -93,6 +93,9 @@ struct _sfio_s;
 #ifndef __FILE_defined
 #define __FILE_defined 1
 #endif
+#ifndef __DEFINED_FILE
+#define __DEFINED_FILE 1
+#endif
 #endif
 
 /* locale stuff */