Fix string escaping in ADD_DEFINITIONS

[AMDmi3]

Recent CMake handles this on its own and does not need a lot of backslashes.

[viewizard]

Wait, since we are requiere cmake 2.8+. https://github.com/viewizard/astromenace/blob/master/CMakeLists.txt#L1
Will this ok for 2.8?

[AMDmi3]

Well it works for me, and AFAIK CMAKE_MINIMUM_REQUIRED makes CMake behave exactly as the version specified, which would mean it both works with latest CMake version and is still compatible with 2.8.

[viewizard]

I am just worry, this could be some kind of surprise, if we have 1 line that need 3.9+ or something. :-D
Sure, 2.8 will be compatible for cmake 3.9, but what about line that supported only by 3.9+ and cmake 2.8?
Btw, any idea, what cmake version have this fixes? 3.9+?

[AMDmi3]

Repeating myself, the assumption is that if it works with CMAKE_MINIMUM_REQUIRED(VERSION 2.8), it's compatible with CMake versions down to 2.8. Without this change though, the build is guaranteed broken with recent CMake versions for sure. Also, CMake 2.8 are long extinct:

[AMDmi3]

Update: the relevant policy is https://cmake.org/cmake/help/v3.12/policy/CMP0005.html.
It was introduced in CMake 2.6. So this change is compatible with and is required for CMake >= 2.6.