message: fix LLVM_AR detection
diff: @@ -63,8 +63,11 @@ ifndef STAN_NO_COMPILER_OPTIMS
  CXXFLAGS_OPTIM ?= -fvectorize -ftree-vectorize -fslp-vectorize -ftree-slp-vectorize -fno-standalone-debug -fstrict-return -fvisibility=hidden -fvisibility-inlines-hidden -funroll-loops
  CXXFLAGS_OPTIM_SUNDIALS ?= -fvectorize -ftree-vectorize -fslp-vectorize -ftree-slp-vectorize -fno-standalone-debug -fstrict-return -fvisibility=hidden -fvisibility-inlines-hidden
  ifeq ($(shell expr $(CXX_MAJOR) \>= 5), 1)
  + ifdef LLVM_AR
  + AR = LLVM_AR
  + else
  ifeq (, $(shell which llvm-ar))
  - $(warning "llvm-ar was not detected in your path, to enable lto optimization please add llvm-ar to your environment PATH")
  + $(warning "llvm-ar was not detected in your path or set via the LLVM_AR flag, to enable lto optimization please add llvm-ar to your environment PATH or add LLVM_AR to your make/local")
  else
  AR = llvm-ar
  CXXFLAGS_FLTO ?= -flto=full -fwhole-program-vtables -fstrict-vtable-pointers -fforce-emit-vtables
  @@ -73,6 +76,7 @@ ifndef STAN_NO_COMPILER_OPTIMS
  endif
  endif
  endif
  + endif
  ifeq (mingw32-g,$(CXX_TYPE))
  else ifeq (gcc,$(CXX_TYPE))
  CXXFLAGS_OPTIM_SUNDIALS ?= -fweb -fivopts -ftree-loop-linear
  