selftests/bpf: add -std=gnu11 to BPF_CFLAGS and CFLAGS

Latest versions of GCC BPF use C23 standard by default. This causes
compilation errors in vmlinux.h due to bool types declarations.

Add -std=gnu11 to BPF_CFLAGS and CFLAGS. This aligns with the version
of the standard used when building the kernel currently [1].

For more details see the discussions at [2] and [3].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile#n465
[2] https://lore.kernel.org/bpf/EYcXjcKDCJY7Yb0GGtAAb7nLKPEvrgWdvWpuNzXm2qi6rYMZDixKv5KwfVVMBq17V55xyC-A1wIjrqG3aw-Imqudo9q9X7D7nLU2gWgbN0w=@pm.me/
[3] https://lore.kernel.org/bpf/20250106202715.1232864-1-ihor.solodrai@pm.me/

CC: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Link: https://lore.kernel.org/r/20250107235813.2964472-1-ihor.solodrai@pm.me
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/Makefile

@@ -41,7 +41,7 @@ srctree := $(patsubst %/,%,$(dir $(srctree)))
 srctree := $(patsubst %/,%,$(dir $(srctree)))
 endif
 
-CFLAGS += -g $(OPT_FLAGS) -rdynamic					\
+CFLAGS += -g $(OPT_FLAGS) -rdynamic -std=gnu11				\
 	  -Wall -Werror -fno-omit-frame-pointer				\
 	  $(GENFLAGS) $(SAN_CFLAGS) $(LIBELF_CFLAGS)			\
 	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
@@ -447,6 +447,7 @@ CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG),$(CLANG_TARGET_ARCH))
 BPF_CFLAGS = -g -Wall -Werror -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN)	\
 	     -I$(INCLUDE_DIR) -I$(CURDIR) -I$(APIDIR)			\
 	     -I$(abspath $(OUTPUT)/../usr/include)			\
+	     -std=gnu11		 					\
 	     -fno-strict-aliasing 					\
 	     -Wno-compare-distinct-pointer-types
 # TODO: enable me -Wsign-compare
@@ -787,9 +788,12 @@ $(OUTPUT)/xdp_features: xdp_features.c $(OUTPUT)/network_helpers.o $(OUTPUT)/xdp
 	$(Q)$(CC) $(CFLAGS) $(filter %.a %.o %.c,$^) $(LDLIBS) -o $@
 
 # Make sure we are able to include and link libbpf against c++.
+CXXFLAGS += $(CFLAGS)
+CXXFLAGS := $(subst -D_GNU_SOURCE=,,$(CXXFLAGS))
+CXXFLAGS := $(subst -std=gnu11,-std=gnu++11,$(CXXFLAGS))
 $(OUTPUT)/test_cpp: test_cpp.cpp $(OUTPUT)/test_core_extern.skel.h $(BPFOBJ)
 	$(call msg,CXX,,$@)
-	$(Q)$(CXX) $(subst -D_GNU_SOURCE=,,$(CFLAGS)) $(filter %.a %.o %.cpp,$^) $(LDLIBS) -o $@
+	$(Q)$(CXX) $(CXXFLAGS) $(filter %.a %.o %.cpp,$^) $(LDLIBS) -o $@
 
 # Benchmark runner
 $(OUTPUT)/bench_%.o: benchs/bench_%.c bench.h $(BPFOBJ)