gnulib: Handle warnings introduced by updated gnulib

- Fix type of size variable in luks2_verify_key()
- Avoid redefinition of SIZE_MAX and ATTRIBUTE_ERROR
- Work around gnulib's int types on older compilers

Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/compiler.h

@@ -30,10 +30,10 @@
 
 /* Does this compiler support compile-time error attributes? */
 #if GNUC_PREREQ(4,3)
-#  define ATTRIBUTE_ERROR(msg) \
+#  define GRUB_ATTRIBUTE_ERROR(msg) \
 	__attribute__ ((__error__ (msg)))
 #else
-#  define ATTRIBUTE_ERROR(msg) __attribute__ ((noreturn))
+#  define GRUB_ATTRIBUTE_ERROR(msg) __attribute__ ((noreturn))
 #endif
 
 #if GNUC_PREREQ(4,4)