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ARROW-5329: [MATLAB] Add support for building MATLAB interface to Feather directly within MATLAB

The MATLAB interface to Feather currently uses CMake to build MEX files. It would be convenient to be able to build the MEX file used by the MATLAB interface to Feather directly using normal MATLAB commands.

This will also provide a workaround for MEX compilation issues when using older CMake versions with newer versions of MATLAB (R2018b and beyond).

The commits in the pull request add a new `build_support` folder to the MATLAB subfolder in the Arrow repository. This `build_support` folder contains two functions called `compile` and `test` that use the `ARROW_HOME` environment variable and the `mex` command to build the necessary MEX files.

For example:

``` matlab
>> cd build_support
>> compile
...lots of output...
>> test
...successfully passed all tests...
```

Thanks to Mark (@mcafaro) for implementing this and working through the initial review in https://github.com/mathworks/arrow/pull/13

Author: Mark Cafaro <mark.cafaro@mathworks.com>
Author: rdmello <rylan.dmello@mathworks.com>
Author: Mark Cafaro <34887852+mcafaro@users.noreply.github.com>

Closes #4321 from rdmello/ARROW-5329 and squashes the following commits:

1a14d83b3 <rdmello> Updating README.md with instructions for building directly using MATLAB
a685bce91 <Mark Cafaro> Switched from MATLAB char arrays to strings
39e8e2a2b <Mark Cafaro> Changed MATLAB MEX compilation to use RPATH only on UNIX systems
a90af5a3d <Mark Cafaro> Changed MATLAB variable names to be more clear
f9e1aeed7 <Mark Cafaro> Fixed MATLAB variable name
03729828d <Mark Cafaro> Added functions for MATLAB build support