Don't hardcode -O2.
This is actually C11 according to clang.

Index: src/Makefile
--- src/Makefile.orig
+++ src/Makefile
@@ -29,7 +29,7 @@ LIBSSLLIBS = $(shell pkg-config --libs openssl)
 LIBSECRETFLAGS =$(shell pkg-config --cflags libsecret-1)
 LIBSECRETLIBS =$(shell pkg-config --libs libsecret-1)
 
-CFLAGS += -std=gnu99 -O2 -Wall -Iinc -Wno-deprecated-declarations \
+CFLAGS += -std=gnu11 -Wall -Iinc -Wno-deprecated-declarations \
 		  $(DEFS) \
 		  $(DBGFLAGS) \
 		  $(GTK3FLAGS) \