build fails with txt2man installed

I was trying to build `mlpack` with the following:

```

cmake -DBUILD_CLI_EXECUTABLES=OFF -DBUILD_TESTS=OFF -DBUILD_PYTHON_BINDINGS=OFF ..

make

```

Although `cmake` succeeded, but `make` was complaining with

```

Scanning dependencies of target mlpack_gitversion

[  1%] Updating gitversion.hpp (if necessary)

-- Found Git: /usr/bin/git (found version "2.7.4") 

-- gitversion.hpp is already up to date.

[  1%] Built target mlpack_gitversion

Scanning dependencies of target mlpack_arma_config

[  2%] Updating arma_config.hpp (if necessary)

-- Regenerating arma_config.hpp.

[  2%] Built target mlpack_arma_config

Scanning dependencies of target man

make[2]: *** No rule to make target '../mlpack_sparse_coding', needed by 'CMakeFiles/man'.  Stop.

CMakeFiles/Makefile2:173: recipe for target 'CMakeFiles/man.dir/all' failed

make[1]: *** [CMakeFiles/man.dir/all] Error 2

Makefile:138: recipe for target 'all' failed

make: *** [all] Error 2

```



I realized that the `CMake code` that is making trouble is inside the `else part` of

```

if (NOT TXT2MAN)

    message(WARNING "txt2man not found; man pages will not be generated.")

  else ()

    . . .

endif ()

```



so, I removed the `txt2man` from my system and tried to build again and it **succeeded** .



**System Information:** Ubuntu 16.04, CMake 3.11, gcc/g++ 7.3

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