message: fix build under macOS Catalina where boost build failed to set correct toolset
diff: @@ -18,8 +18,12 @@ BOOST_PROGRAM_OPTIONS_LIB=$(BOOST)/stage/lib/libboost_program_options*.a
  BOOST_BUILD_OPTS= address-model=$(WINBITNESS)
  else
  BOOST_PROGRAM_OPTIONS_LIB=$(BOOST)/stage/lib/libboost_program_options.a
  +ifeq ($(OS),Darwin)
  +BOOST_BUILD_OPTS ?= toolset=$(CXX_TYPE)
  +else
  BOOST_BUILD_OPTS=
  endif
  +endif
  .PRECIOUS: bin/print$(EXE) bin/stansummary$(EXE) bin/diagnose$(EXE)
  bin/print$(EXE) bin/stansummary$(EXE) bin/diagnose$(EXE) : CPPFLAGS_MPI =
  