x: Fix x_fopencookie() for some systems.

Some OSs require _GNU_SOURCE for cookie_io_functions_t, which made the
previous approach of just using #define to access that struct
unworkable.  Instead define our own data types and use a proper wrapper.

Author: lorinder

src/csnip/x.h

@@ -79,13 +79,7 @@ int asprintf(char** strp, const char* fmt, ...);
 int csnip_x_asprintf_imp(char** strp, const char* format, ...);
 
 /**	Wrapper for system fopencookie() or funopen(). */
-#if defined(CSNIP_CONF__HAVE_FOPENCOOKIE)
-typedef cookie_io_functions_t csnip_x_cookie_io_functions_t;
-
-FILE* csnip_x_fopencookie(void* __restrict__ cookie,
-			const char* __restrict__ mode,
-			csnip_x_cookie_io_functions_t funcs);
-#elif defined(CSNIP_CONF__HAVE_FUNOPEN)
+#if defined(CSNIP_CONF__HAVE_FOPENCOOKIE) || defined(CSNIP_CONF__HAVE_FUNOPEN)
 typedef csnip_x_ssize_t csnip_x_cookie_read_function_t(
 			void* cookie,
 			char* buf,

src/csnip/x/fopencookie.c

@@ -12,7 +12,13 @@ FILE* x_fopencookie(void* restrict cookie,
 			const char* restrict mode,
 			x_cookie_io_functions_t io_funcs)
 {
-	return fopencookie(cookie, mode, io_funcs);
+	cookie_io_functions_t io_funcs2 = {
+		.read = io_funcs.read,
+		.write = io_funcs.write,
+		.seek = io_funcs.seek,
+		.close = io_funcs.close,
+	};
+	return fopencookie(cookie, mode, io_funcs2);
 }
 
 #elif defined(CSNIP_CONF__HAVE_FUNOPEN)