message: do not allow precompiled headers with clang by default
diff: @@ -108,11 +108,15 @@ endif
  STAN_FLAGS=$(STAN_FLAG_THREADS)$(STAN_FLAG_MPI)$(STAN_FLAG_OPENCL)
  ifeq ($(OS),Windows_NT)
  +ifeq (clang,$(CXX_TYPE))
  +PRECOMPILED_HEADERS ?= false
  +else
  ifeq ($(shell expr $(CXX_MAJOR) \>= 8), 1)
  PRECOMPILED_HEADERS ?= true
  else
  PRECOMPILED_HEADERS ?= false
  endif
  +endif
  else
  PRECOMPILED_HEADERS ?= true
  endif
  