message: Fixed building doc with only llcm13 installed
diff: @@ -130,6 +130,8 @@ else ifeq ($(ENABLE_LLVM), 1)
  else ifneq (, $(shell which llvm-config-13 2>/dev/null))
  LLVM_VERSION := $(shell llvm-config-13 --version)
  compilers += llvm
  + # need force LLVM_SYS_120_PREFIX, or llvm_sys will not build in the case
  + export LLVM_SYS_120_PREFIX = $(shell llvm-config-13 --prefix)
  else ifneq (, $(shell which llvm-config-12 2>/dev/null))
  LLVM_VERSION := $(shell llvm-config-12 --version)
  compilers += llvm
  