update to use logic to check compiler version for c++17 compatability

make/compiler_flags

@@ -120,35 +120,45 @@ INC_GTEST ?= -I $(GTEST)/include -I $(GTEST)
 CPPFLAGS_BOOST ?= -DBOOST_DISABLE_ASSERTS
 CPPFLAGS_SUNDIALS ?= -DNO_FPRINTF_OUTPUT $(CPPFLAGS_OPTIM_SUNDIALS) $(CXXFLAGS_FLTO_SUNDIALS)
 #CPPFLAGS_GTEST ?=
-
+HAS_CXX17 := false
 ifeq ($(CXX_TYPE), gcc)
   GCC_GE_73 := $(shell [ $(CXX_MAJOR) -gt 7 -o \( $(CXX_MAJOR) -eq 7 -a $(CXX_MINOR) -ge 1 \) ] && echo true)
   ifeq ($(GCC_GE_73),true)
     CXXFLAGS_LANG ?= -std=c++17
     CXXFLAGS_STANDARD ?= c++17
+    HAS_CXX17 := true
   endif
 else ifeq ($(CXX_TYPE), clang)
   CLANG_GE_5 := $(shell [ $(CXX_MAJOR) -gt 5 -o \( $(CXX_MAJOR) -eq 5 -a $(CXX_MINOR) -ge 0 \) ] && echo true)
   ifeq ($(CLANG_GE_5),true)
     CXXFLAGS_LANG ?= -std=c++17
     CXXFLAGS_STANDARD ?= c++17
+    HAS_CXX17 := true
   endif
 else ifeq ($(CXX_TYPE), mingw32-gcc)
   MINGW_GE_50 := $(shell [ $(CXX_MAJOR) -gt 5 -o \( $(CXX_MAJOR) -eq 5 -a $(CXX_MINOR) -ge 0 \) ] && echo true)
   ifeq ($(MINGW_GE_50),true)
     CXXFLAGS_LANG ?= -std=c++17
     CXXFLAGS_STANDARD ?= c++17
+    HAS_CXX17 := true
   endif
 else ifeq ($(STAN_USE_CPP14),)
   CXXFLAGS_LANG ?= -std=c++17
   CXXFLAGS_STANDARD ?= c++17
+  HAS_CXX17 := true
 else
   $(warning "Because STAN_USE_CPP14 is set C++14 standard is requested. This will not be possible in the next release!")
   CXXFLAGS_LANG ?= -std=c++1y
   CXXFLAGS_STANDARD ?= c++1y
+  HAS_CXX17 := true
 endif
 
-
+ifeq ($(HAS_CXX17), false)
+  $(warning "Because STAN_USE_CPP14 is set C++14 standard is requested. This will not be possible in the next release!")
+  CXXFLAGS_LANG ?= -std=c++1y
+  CXXFLAGS_STANDARD ?= c++1y
+  HAS_CXX17 := true
+endif
 #CXXFLAGS_BOOST ?=
 CXXFLAGS_SUNDIALS ?= -pipe $(CXXFLAGS_OPTIM_SUNDIALS) $(CPPFLAGS_FLTO_SUNDIALS)
 #CXXFLAGS_GTEST