Update build instructions for CentOS7, tweaks to CMake script

[swertz]

Note: I've realized there is some overlap with #19 - if that is merged first I can rebase.

Addresses #18 , although I did not encounter some of the issues mentioned there (forcing compiler version, linking to pthread, ... because of different devtoolset or cmake version?).

• Remove build folder to let cmake handle it and make it possible to transparently rm -r build to start a clean build
• Bump cmake version - I've chosen 3.11 which is reasonably new; even centOS7 EPEL has 3.17 anyway.
• Make some changes in cmake to "modern" cmake style, but some is still "old" style
• README: add requirements and instructions for centOS7 - some of the cmake changes were required to build there (e.g. adding -ldl), I hope that didn't break the builds on modern systems...

[swertz]

Updated with a bugfix in CommonDef.h:: CAENVME_API does not name a type, most likely what was meant was CVErrorCodes...?