Don't clobber toolchain defaults.

https://bugs.gentoo.org/890278
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,4 +1,4 @@
-AM_CFLAGS = -Wall -Wextra -D_FORTIFY_SOURCE=2 $(PIE_CFLAGS) $(RELRO_CFLAGS)
+AM_CFLAGS = -Wall -Wextra $(PIE_CFLAGS) $(RELRO_CFLAGS)
 ACLOCAL_AMFLAGS = -I aclocal
 
 root_sbindir = $(ROOTSBINDIR)