temporary change back to C++17

Makefile.linux

@@ -24,7 +24,8 @@ else ifeq ($(shell expr match ${CXXVER} "[7-9]"),1) # gcc 7 - 9
 	NEEDED_CXXFLAGS += -std=c++17
 	LDLIBS = -latomic
 else ifeq ($(shell expr match ${CXXVER} "1[0-9]"),2) # gcc 10 - 19
-	NEEDED_CXXFLAGS += -std=c++20
+#	NEEDED_CXXFLAGS += -std=c++20
+	NEEDED_CXXFLAGS += -std=c++17
 	LDLIBS = -latomic
 else # not supported
 $(error Compiler too old)