Index: sys/amd64/include/efi.h
===================================================================
--- sys/amd64/include/efi.h
+++ sys/amd64/include/efi.h
@@ -36,8 +36,14 @@
  * XXX: from gcc 6.2 manual:
  * Note, the ms_abi attribute for Microsoft Windows 64-bit targets
  * currently requires the -maccumulate-outgoing-args option.
+ *
+ * Avoid EFIABI_ATTR declarations for compilers that don't support it.
+ * GCC support began in version 4.4.
  */
+#if defined(__clang__) || defined(__INTEL_COMPILER) || !defined(__GNUC__) || \
+    (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 4)
 #define	EFIABI_ATTR	__attribute__((ms_abi))
+#endif
 
 #ifdef _KERNEL
 struct uuid;
Index: sys/sys/efi.h
===================================================================
--- sys/sys/efi.h
+++ sys/sys/efi.h
@@ -122,6 +122,9 @@
 	uint32_t	__res;
 };
 
+#ifdef _KERNEL
+
+#ifdef EFIABI_ATTR
 struct efi_rt {
 	struct efi_tblhdr rt_hdr;
 	efi_status	(*rt_gettime)(struct efi_tm *, struct efi_tmcap *)
@@ -144,6 +147,7 @@
 	efi_status	(*rt_reset)(enum efi_reset, efi_status, u_long,
 	    efi_char *) EFIABI_ATTR;
 };
+#endif
 
 struct efi_systbl {
 	struct efi_tblhdr st_hdr;
@@ -163,7 +167,6 @@
 	uint64_t	st_cfgtbl;
 };
 
-#ifdef _KERNEL
 extern vm_paddr_t efi_systbl_phys;
 #endif	/* _KERNEL */