make vstart failed with ./do_cmake.sh -DWITH_JAEGER=ON

 CMake Error at /usr/share/cmake-3.10/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
7266  Could NOT find FLEX (missing: FLEX_EXECUTABLE)
7267 Call Stack (most recent call first):
7268  /usr/share/cmake-3.10/Modules/FindPackageHandleStandardArgs.cmake:378 (_FPHSA_FAILURE_MESSAGE)
7269  /usr/share/cmake-3.10/Modules/FindFLEX.cmake:217 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
7270  compiler/cpp/CMakeLists.txt:25 (find_package)
7271 -- Configuring incomplete, errors occurred!
7272 See also "/ceph/build/external/thrift/CMakeFiles/CMakeOutput.log".
7273 See also "/ceph/build/external/thrift/CMakeFiles/CMakeError.log".
7274 make[3]: *** [external/thrift/src/thrift-stamp/thrift-configure] Error 1
7275 src/CMakeFiles/thrift.dir/build.make:106: recipe for target 'external/thrift/src/thrift-stamp/thrift-configure' failed
7276 CMakeFiles/Makefile2:1384: recipe for target 'src/CMakeFiles/thrift.dir/all' failed
7277 make[2]: *** [src/CMakeFiles/thrift.dir/all] Error 2
7278 make[2]: *** Waiting for unfinished jobs....