Update build docs to indicate cmake 3.x is required

[derek-gerstmann]

Many Linux distros (eg CentOS 7.x) still ship with an old cmake 2.8 as default and the latest CMakeModules.txt updates use cmake 3.x functions, which cause strange errors to be reported on older versions of cmake.

Update the build docs to indicate cmake 3.x is required.

Thanks!!!

-- dg

[febret]

Hi, do you have a specific module failing & the error CMake spits out? The travis build uses CMake 2.8.11 I believe (but only builds the core omegalib, no modules).

I was planning to push the required version up to 3.5 (https://github.com/uic-evl/omegalib/blob/master/CMakeLists.txt#L2) but I would also need to find a newer version of cmake to install on travis in the setup script (https://github.com/uic-evl/omegalib/blob/master/external/setup-travis.sh) or the CI builds will fail..

[derek-gerstmann]

I believe most of the errors were from the use of the DIRECTORY component specified as a parameter to get_filename_component. CMAKE v2.8x used PATH, and CMAKE v3.x introduced DIRECTORY:

https://cmake.org/cmake/help/v3.0/command/get_filename_component.html