package/bash: fix build with host GCC 15

bash uses K&R function declarations which have been removed in C23.

Since part of the build process (like the mkbuiltins helper) is written
in C, building bash now fails on hosts with GCC 15 (which defaults to
C23).

Since properly fixing this on the source code level is a larger
endeavor, just set the C standard to an old enough version for now.

Signed-off-by: Florian Larysch <fl@n621.de>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/bash/bash.mk

@@ -26,6 +26,10 @@ BASH_CONF_ENV += \
 	bash_cv_func_sigsetjmp=present \
 	bash_cv_printf_a_format=yes
 
+ifeq ($(BR2_HOST_GCC_AT_LEAST_15),y)
+BASH_CONF_ENV += CFLAGS_FOR_BUILD="$(HOST_CFLAGS) -std=gnu17"
+endif
+
 # The static build needs some trickery
 ifeq ($(BR2_STATIC_LIBS),y)
 BASH_CONF_OPTS += --enable-static-link