cygwin: workaround GCC 6 changes

GCC 6 includes a number of new warnings which cause Cygwin to either not
compile, or not work properly even if said warnings are ignored:

https://cygwin.com/ml/cygwin-developers/2017-09/msg00000.html
https://gcc.gnu.org/gcc-6/porting_to.html

For now, we use the flags necessary to revert to GCC 5 behaviour until we
can fix the code properly.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

winsup/cygwin/Makefile.in

@@ -73,11 +73,11 @@ CRT0:=$(cygwin_build)/crt0.o
 #
 MT_SAFE:=@MT_SAFE@
 CCEXTRA=
-COMMON_CFLAGS=-MMD ${$(*F)_CFLAGS} -Werror -fmerge-constants -ftracer $(CCEXTRA)
+COMMON_CFLAGS=-MMD ${$(*F)_CFLAGS} -Werror -fno-delete-null-pointer-checks -fmerge-constants -ftracer $(CCEXTRA)
 ifeq ($(target_cpu),x86_64)
 COMMON_CFLAGS+=-mcmodel=small
 endif
-COMPILE.cc+=${COMMON_CFLAGS}
+COMPILE.cc+=${COMMON_CFLAGS} -std=gnu++98
 COMPILE.c+=${COMMON_CFLAGS}
 
 AR:=@AR@

winsup/cygwin/crt0.c

@@ -20,7 +20,9 @@ void
 mainCRTStartup ()
 {
 #ifdef __i386__
+#pragma GCC diagnostic ignored "-Wframe-address"
   (void)__builtin_return_address(1);
+#pragma GCC diagnostic pop
   asm volatile ("andl $-16,%%esp" ::: "%esp");
 #endif
 

winsup/cygwin/init.cc

@@ -23,7 +23,9 @@ static void WINAPI
 threadfunc_fe (VOID *arg)
 {
 #ifndef __x86_64__
+#pragma GCC diagnostic ignored "-Wframe-address"
   (void)__builtin_return_address(1);
+#pragma GCC diagnostic pop
   asm volatile ("andl $-16,%%esp" ::: "%esp");
 #endif
   _cygtls::call ((DWORD (*)  (void *, void *)) TlsGetValue (_my_oldfunc), arg);