add openfst.cmake file (#707)

1. When compiling locally, openfst is missing.so add this file to the sherpa-onnx/cmake folder

+# Copyright (c)  2020  Xiaomi Corporation (author: Fangjun Kuang)
+
+function(download_openfst)
+  include(FetchContent)
+
+  set(openfst_URL  "https://github.com/kkm000/openfst/archive/refs/tags/win/1.6.5.1.tar.gz")
+  set(openfst_URL2  "https://huggingface.co/csukuangfj/kaldi-hmm-gmm-cmake-deps/resolve/main/openfst-win-1.6.5.1.tar.gz")
+  set(openfst_HASH "SHA256=02c49b559c3976a536876063369efc0e41ab374be1035918036474343877046e")
+
+  # If you don't have access to the Internet,
+  # please pre-download it
+  set(possible_file_locations
+    $ENV{HOME}/Downloads/openfst-win-1.6.5.1.tar.gz
+    ${CMAKE_SOURCE_DIR}/openfst-win-1.6.5.1.tar.gz
+    ${CMAKE_BINARY_DIR}/openfst-win-1.6.5.1.tar.gz
+    /tmp/openfst-win-1.6.5.1.tar.gz
+    /star-fj/fangjun/download/github/openfst-win-1.6.5.1.tar.gz
+  )
+
+  foreach(f IN LISTS possible_file_locations)
+    if(EXISTS ${f})
+      set(openfst_URL  "${f}")
+      file(TO_CMAKE_PATH "${openfst_URL}" openfst_URL)
+      set(openfst_URL2)
+      break()
+    endif()
+  endforeach()
+
+  set(HAVE_BIN OFF CACHE BOOL "" FORCE)
+  set(HAVE_SCRIPT ON CACHE BOOL "" FORCE)
+  set(HAVE_COMPACT OFF CACHE BOOL "" FORCE)
+  set(HAVE_COMPRESS OFF CACHE BOOL "" FORCE)
+  set(HAVE_CONST OFF CACHE BOOL "" FORCE)
+  set(HAVE_FAR OFF CACHE BOOL "" FORCE)
+  set(HAVE_GRM OFF CACHE BOOL "" FORCE)
+  set(HAVE_PDT OFF CACHE BOOL "" FORCE)
+  set(HAVE_MPDT OFF CACHE BOOL "" FORCE)
+  set(HAVE_LINEAR OFF CACHE BOOL "" FORCE)
+  set(HAVE_LOOKAHEAD OFF CACHE BOOL "" FORCE)
+  set(HAVE_NGRAM OFF CACHE BOOL "" FORCE)
+  set(HAVE_PYTHON OFF CACHE BOOL "" FORCE)
+  set(HAVE_SPECIAL OFF CACHE BOOL "" FORCE)
+
+  if(NOT WIN32)
+    FetchContent_Declare(openfst
+      URL
+        ${openfst_URL}
+        ${openfst_URL2}
+      URL_HASH          ${openfst_HASH}
+      PATCH_COMMAND
+        sed -i.bak s/enable_testing\(\)//g "src/CMakeLists.txt" &&
+        sed -i.bak s/add_subdirectory\(test\)//g "src/CMakeLists.txt" &&
+        sed -i.bak /message/d "src/script/CMakeLists.txt"
+        # sed -i.bak s/add_subdirectory\(script\)//g "src/CMakeLists.txt" &&
+        # sed -i.bak s/add_subdirectory\(extensions\)//g "src/CMakeLists.txt"
+    )
+  else()
+    FetchContent_Declare(openfst
+      URL               ${openfst_URL}
+      URL_HASH          ${openfst_HASH}
+    )
+  endif()
+
+  FetchContent_GetProperties(openfst)
+  if(NOT openfst_POPULATED)
+    message(STATUS "Downloading openfst from ${openfst_URL}")
+    FetchContent_Populate(openfst)
+  endif()
+  message(STATUS "openfst is downloaded to ${openfst_SOURCE_DIR}")
+  add_subdirectory(${openfst_SOURCE_DIR} ${openfst_BINARY_DIR} EXCLUDE_FROM_ALL)
+  set(openfst_SOURCE_DIR ${openfst_SOURCE_DIR} PARENT_SCOPE)
+
+  # Rename libfst.so.6 to libkaldifst_fst.so.6 to avoid potential conflicts
+  # when kaldifst is installed.
+  set_target_properties(fst PROPERTIES OUTPUT_NAME "kaldifst_fst")
+
+  install(TARGETS fst
+    DESTINATION lib
+  )
+
+  if(KALDIFST_BUILD_PYTHON)
+    set_target_properties(fstscript PROPERTIES OUTPUT_NAME "kaldifst_fstscript")
+    install(TARGETS fstscript
+      DESTINATION lib
+    )
+  endif()
+endfunction()
+
+download_openfst()