[PATCH] perf build: Fix broken feature check for llvm due to C++ standard

[Thomas Richter]

Perf build auto-detects features and packages already installed
for its build. This is done in directory tools/build/feature. This
directory contains small sample programs. When they successfully
compile the necessary prereqs in form of libraries and header
files are present.

Such a check is also done for llvm. And this check fails.
Fix this and update to the latest C++ standard.

Output before:
 # rm -f ./test-llvm.bin; make test-llvm.bin; ./test-llvm.bin
 g++  -MD -Wall -Werror -o test-llvm.bin test-llvm.cpp \
	 	> test-llvm.make.output 2>&1 -std=gnu++14 \
	-I/usr/include 		\
	-L/usr/lib64		\
	-lLLVM-16		\
			\
	> test-llvm.make.output 2>&1

 make: *** [Makefile:343: test-llvm.bin] Error 1
 -bash: ./test-llvm.bin: No such file or directory
 #

Output after:
 # rm -f ./test-llvm.bin; make test-llvm.bin; ./test-llvm.bin
 g++  -MD -Wall -Werror -o test-llvm.bin test-llvm.cpp \
	 	> test-llvm.make.output 2>&1 -std=gnu++17 \
	-I/usr/include 		\
	-L/usr/lib64		\
	-lLLVM-16		\
			\
	> test-llvm.make.output 2>&1
 Hello World!
 #

Signed-off-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>
---
 tools/build/feature/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
index 0f0aa9b7d7b5..f8db69654791 100644
--- a/tools/build/feature/Makefile
+++ b/tools/build/feature/Makefile
@@ -340,7 +340,7 @@ $(OUTPUT)test-jvmti-cmlr.bin:
 	$(BUILD)
 
 $(OUTPUT)test-llvm.bin:
-	$(BUILDXX) -std=gnu++14 				\
+	$(BUILDXX) -std=gnu++17					\
 		-I$(shell $(LLVM_CONFIG) --includedir) 		\
 		-L$(shell $(LLVM_CONFIG) --libdir)		\
 		$(shell $(LLVM_CONFIG) --libs Core BPF)		\
-- 
2.41.0