Improve the build system with cmake's ExrternalProject_Add

[ggarra13]

Currently, the build instructions rely on packagest from the Linux distribution, which is fine. However, the other libraries xStudio depends on (OpenEXR, etc) are to be built manually and installed in /usr/local. This is a terrible practice as it will lead to all sort of library misshaps as soon as there are library updates.
cmake provides an instruction called ExternalProject_Add() to download, build and (locally) install the libraries as part of the cmake project.
Please make use of that.

[tedwaine]

Hi - yes this is a good point. Our documentation should be clearer about this: The build instructions provided are minimal, intended to be taken 'as-is' and serve only as a guideline. Anyone building and installing multiple versions of OpenEXR for other software, say, should be able to manage this themselves by overriding the prefix path and so-on. Providing convenient build with dependency version management for multiple Linux distros is beyond our resources at the moment.