don't break with old compilers and -DGNU_EFI_USE_MS_ABI
It's entirely legitimate to request GNU_EFI_USE_MS_ABI even if the current
compiler doesn't support it, and gnu-efi should transparently fall back to
using legacy techniques to set the calling convention.  We don't get type
checking, but at least it will still compile.

Adapted from gnu-efi

Author: Steve Langasek <steve.langasek@ubuntu.com>
Upstream-Status: Pending

Index: syslinux-6.03/efi64/include/efi/x86_64/efibind.h
===================================================================
--- syslinux-6.03.orig/efi64/include/efi/x86_64/efibind.h
+++ syslinux-6.03/efi64/include/efi/x86_64/efibind.h
@@ -25,8 +25,6 @@ Revision History
 #if defined(GNU_EFI_USE_MS_ABI)
     #if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))
         #define HAVE_USE_MS_ABI 1
-    #else
-        #error Compiler is too old for GNU_EFI_USE_MS_ABI
     #endif
 #endif
 
Index: syslinux-6.03/gnu-efi/gnu-efi-3.0/inc/x86_64/efibind.h
===================================================================
--- syslinux-6.03.orig/gnu-efi/gnu-efi-3.0/inc/x86_64/efibind.h
+++ syslinux-6.03/gnu-efi/gnu-efi-3.0/inc/x86_64/efibind.h
@@ -25,8 +25,6 @@ Revision History
 #if defined(GNU_EFI_USE_MS_ABI)
     #if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))
         #define HAVE_USE_MS_ABI 1
-    #else
-        #error Compiler is too old for GNU_EFI_USE_MS_ABI
     #endif
 #endif