AM_CPPFLAGS = -I$(top_srcdir)/include $(PLATFORM_CPPFLAGS) \
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE
-AM_CFLAGS = -std=c99 $(PLATFORM_CFLAGS) -fno-common \
+# Note: -std=gnu99 instead of -std=c99 is needed for unnamed structs and unions,
+# which are in C11 but not C99. But we can't yet actually use -std=c11 because
+# we want to support older versions of gcc.
+AM_CFLAGS = -std=gnu99 $(PLATFORM_CFLAGS) -fno-common \
-Wmissing-prototypes -Wstrict-prototypes \
-Wundef -Wno-pointer-sign