--- a/Rules.make
+++ b/Rules.make
@@ -26,2 +26,7 @@ CFLAGS += -fno-stack-protector
 endif
+ifeq ($(call cc-option-yn, -std=gnu90),y)
+# In gcc-5+ c99 changed inline rules by forbidding
+# multiple extern definitions. Fall back to gnu90
+CFLAGS += -std=gnu90
+endif
 
--- a/silo/Makefile
+++ b/silo/Makefile
@@ -6,2 +6,7 @@ UFSDEPS=
 CFLAGS=-O2 -Wall -I. -I../include
+ifeq ($(call cc-option-yn, -std=gnu90),y)
+# In gcc-5+ c99 changed inline rules by forbidding
+# multiple extern definitions. Fall back to gnu90
+CFLAGS += -std=gnu90
+endif