update makefile to use gcc 7 on archlinux

Makefile.linux

@@ -19,7 +19,7 @@ else ifeq ($(shell expr match ${CXXVER} "4\.[7-9]"),3) # >= 4.7
 	NEEDED_CXXFLAGS += -std=c++11 -D_GLIBCXX_USE_NANOSLEEP=1
 else ifeq ($(shell expr match ${CXXVER} "4\.6"),3) # = 4.6
 	NEEDED_CXXFLAGS += -std=c++0x
-else ifeq ($(shell expr match ${CXXVER} "[5-6]\.[0-9]"),3) # gcc >= 5.0
+else ifeq ($(shell expr match ${CXXVER} "[5-7]\.[0-9]"),3) # gcc >= 5.0
 	NEEDED_CXXFLAGS += -std=c++11
 else # not supported
 	$(error Compiler too old)