diff --git a/Makefile.am b/Makefile.am
index 51c6d33af..a5e07cfa6 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -2533,7 +2533,8 @@ if ARCH_X86_64
 efi_cflags += \
 	-mno-red-zone \
 	-DEFI_FUNCTION_WRAPPER \
-	-DGNU_EFI_USE_MS_ABI
+	-DGNU_EFI_USE_MS_ABI \
+	-std=gnu90
 endif
 
 efi_ldflags = \