Define _GNU_SOURCE for a declaration of strptime

This is needed for compatibility with future C compilers which reject
implicit function declarations by default.  Without _GNU_SOURCE (or a
similar feature test macro), <time.h> does not declare the strptime
function, and compilation can fail.

Submitted upstream:

  <https://lore.kernel.org/linux-efi/87fsdhllhk.fsf@oldenburg.str.redhat.com/>

diff --git a/Make.rules b/Make.rules
index 903a5a4..d4de1ef 100644
--- a/Make.rules
+++ b/Make.rules
@@ -14,7 +14,7 @@ else
 $(error unknown architecture $(ARCH))
 endif
 INCDIR	   = -I$(TOPDIR)include/ -I/usr/include/efi -I/usr/include/efi/$(ARCH) -I/usr/include/efi/protocol
-CPPFLAGS   = -DCONFIG_$(ARCH)
+CPPFLAGS   = -DCONFIG_$(ARCH) -D_GNU_SOURCE
 CFLAGS	   = -O2 -g $(ARCH3264) -fpic -Wall -fshort-wchar -fno-strict-aliasing -fno-merge-constants -fno-stack-protector -ffreestanding -fno-stack-check
 LDFLAGS	   = -nostdlib
 CRTOBJ		= crt0-efi-$(ARCH).o