message: Makefile: allow dropping -Werror from cxxflags
  gcc-6.2.0 generates a couple of warnings when compiling ispc, so
  allow to drop '-Werror' from the cxxflags. It is anyhow a bad idea
  to enforce that flag as it is unclear what future compiler
  versions will warn about.
  Ref:
diff: @@ -162,11 +162,12 @@ endif
  CXX=clang++
  OPT=-O2
  +WERROR=-Werror
  CXXFLAGS=$(OPT) $(LLVM_CXXFLAGS) -I. -Iobjs/ -I$(CLANG_INCLUDE) \
  $(LLVM_VERSION_DEF) \
  -Wall \
  -DBUILD_DATE="\"$(BUILD_DATE)\"" -DBUILD_VERSION="\"$(BUILD_VERSION)\"" \
  - -Wno-sign-compare -Wno-unused-function -Werror
  + -Wno-sign-compare -Wno-unused-function $(WERROR)
  # if( !($(LLVM_VERSION) == LLVM_3_2 || $(LLVM_VERSION) == LLVM_3_3 || $(LLVM_VERSION) == LLVM_3_4))
  ifeq (,$(filter $(LLVM_VERSION), LLVM_3_2 LLVM_3_3 LLVM_3_4))
  