GCC 15 defaults to -std=gnu23. While most of the kernel builds with -std=gnu11,
some of it forgets to pass that flag. Hack in CSTD_FLAG to pass -std=gnu11
everywhere.
 
https://lore.kernel.org/linux-kbuild/20241119044724.GA2246422@thelio-3990X/
--- a/Makefile	2025-05-03 15:20:32.416275719 -0400
+++ b/Makefile	2025-05-03 15:31:25.208482201 -0400
@@ -428,6 +428,8 @@ export KCONFIG_CONFIG
 # SHELL used by kbuild
 CONFIG_SHELL := sh
 
+CSTD_FLAG := -std=gnu11
+
 HOST_LFS_CFLAGS := $(shell getconf LFS_CFLAGS 2>/dev/null)
 HOST_LFS_LDFLAGS := $(shell getconf LFS_LDFLAGS 2>/dev/null)
 HOST_LFS_LIBS := $(shell getconf LFS_LIBS 2>/dev/null)
@@ -449,7 +451,7 @@ HOSTRUSTC = rustc
 HOSTPKG_CONFIG	= pkg-config
 
 KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
-			 -O2 -fomit-frame-pointer -std=gnu11
+			 -O2 -fomit-frame-pointer $(CSTD_FLAG)
 KBUILD_USERCFLAGS  := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
 KBUILD_USERLDFLAGS := $(USERLDFLAGS)
 
@@ -557,7 +559,7 @@ LINUXINCLUDE    := \
 KBUILD_AFLAGS   := -D__ASSEMBLY__ -fno-PIE
 
 KBUILD_CFLAGS :=
-KBUILD_CFLAGS += -std=gnu11
+KBUILD_CFLAGS += $(CSTD_FLAG)
 KBUILD_CFLAGS += -fshort-wchar
 KBUILD_CFLAGS += -funsigned-char
 KBUILD_CFLAGS += -fno-common
@@ -606,7 +608,7 @@ export CPP AR NM STRIP OBJCOPY OBJDUMP R
 export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
 export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
 export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
-export KBUILD_USERCFLAGS KBUILD_USERLDFLAGS
+export KBUILD_USERCFLAGS KBUILD_USERLDFLAGS CSTD_FLAG
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
 export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
--- a/arch/arm64/kernel/vdso32/Makefile	2025-05-03 15:21:13.032492816 -0400
+++ b/arch/arm64/kernel/vdso32/Makefile	2025-05-03 15:32:32.502298323 -0400
@@ -65,7 +65,7 @@ VDSO_CFLAGS += -Wall -Wundef -Wstrict-pr
                -fno-strict-aliasing -fno-common \
                -Werror-implicit-function-declaration \
                -Wno-format-security \
-               -std=gnu11
+               $(CSTD_FLAG)
 VDSO_CFLAGS  += -O2
 # Some useful compiler-dependent flags from top-level Makefile
 VDSO_CFLAGS += $(call cc32-option,-Wno-pointer-sign)
--- a/drivers/firmware/efi/libstub/Makefile	2025-05-03 15:22:44.523804022 -0400
+++ b/drivers/firmware/efi/libstub/Makefile	2025-05-03 15:34:35.211231426 -0400
@@ -7,7 +7,7 @@
 #
 
 # non-x86 reuses KBUILD_CFLAGS, x86 does not
-cflags-y			:= $(KBUILD_CFLAGS)
+cflags-y			:= $(KBUILD_CFLAGS) $(CSTD_FLAG)
 
 cflags-$(CONFIG_X86_32)		:= -march=i386
 cflags-$(CONFIG_X86_64)		:= -mcmodel=small
@@ -18,7 +18,7 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__K
 				   $(call cc-disable-warning, address-of-packed-member) \
 				   $(call cc-disable-warning, gnu) \
 				   -fno-asynchronous-unwind-tables \
-				   $(CLANG_FLAGS)
+				   $(CLANG_FLAGS) $(CSTD_FLAG)
 
 # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
 # disable the stackleak plugin
@@ -40,7 +40,7 @@ KBUILD_CFLAGS			:= $(subst $(CC_FLAGS_FT
 				   -ffreestanding \
 				   -fno-stack-protector \
 				   $(call cc-option,-fno-addrsig) \
-				   -D__DISABLE_EXPORTS
+				   -D__DISABLE_EXPORTS $(CSTD_FLAG)
 
 #
 # struct randomization only makes sense for Linux internal types, which the EFI
--- a/arch/x86/boot/compressed/Makefile	2025-05-03 15:23:42.863989230 -0400
+++ b/arch/x86/boot/compressed/Makefile	2025-05-03 15:35:41.268113485 -0400
@@ -33,7 +33,7 @@ targets := vmlinux vmlinux.bin vmlinux.b
 # case of cross compiling, as it has the '--target=' flag, which is needed to
 # avoid errors with '-march=i386', and future flags may depend on the target to
 # be valid.
-KBUILD_CFLAGS := -m$(BITS) -O2 $(CLANG_FLAGS)
+KBUILD_CFLAGS := -m$(BITS) -O2 $(CLANG_FLAGS) $(CSTD_FLAG)
 KBUILD_CFLAGS += -std=gnu11
 KBUILD_CFLAGS += -fno-strict-aliasing -fPIE
 KBUILD_CFLAGS += -Wundef