fixup clang version detection

Author: CamJN

Makefile-variables

@@ -27,11 +27,14 @@ CPPFLAGS    := -MMD -MP
 
 COMPILER_VERSION		:= $(shell $(CXX) --version | grep version | grep -o -m 1 "[0-9]\+\.[0-9]\+\.*[0-9]*" | head -n 1)
 COMPILER_VERSION_NUMBER		:= $(shell echo $(COMPILER_VERSION) | sed -e 's/\.\([0-9][0-9]\)/\1/g' -e 's/\.\([0-9]\)/0\1/g' -e 's/^[0-9]\{3,4\}$$/&00/')
+CLANG_16_OR_MORE		:= $(shell expr $(COMPILER_VERSION_NUMBER) \>= 160000)
 CLANG_13_OR_MORE		:= $(shell expr $(COMPILER_VERSION_NUMBER) \>= 130106)
-ifneq ($(CLANG_13_OR_MORE),0)
+ifneq ($(CLANG_16_OR_MORE),0)
 # supported: c++11, c++14, c++17, c++20, c++23
-# future: c++2c
+# future: c++2c/c++26
 CXXFLAGS ?= --std=c++23 -O3 -Iinclude $(shell pkg-config --cflags getargv)
+else ifneq ($(CLANG_13_OR_MORE),0)
+CXXFLAGS ?= --std=c++20 -O3 -Iinclude $(shell pkg-config --cflags getargv)
 else
 CXXFLAGS ?= --std=c++17 -O3 -Iinclude $(shell pkg-config --cflags getargv)
 endif