C++20 for clang >= 16

Makefile.bsd

@@ -15,7 +15,8 @@ CXXVER := $(shell $(CXX) -dumpversion)
 ifeq (${CXXVER}, "4.2.1") # older clang always returned 4.2.1
 	$(error Compiler too old)
 else ifeq ($(shell expr match ${CXXVER} "1[6-9]"),2) # clang 16 - 19
-	NEEDED_CXXFLAGS += -std=c++20
+	NEEDED_CXXFLAGS = -std=c++20
 else
 	NEEDED_CXXFLAGS = -std=c++17
 endif
+