CMake build no longer uses generator expressions in defines

[islas]

TYPE: bug fix

KEYWORDS: cmake, compilation

SOURCE: internal

DESCRIPTION OF CHANGES:
Problem:
The use of generator expressions in the defines compacts the logic neatly but removes the ability to evaluate these conditionals at configuration time. As such, assumptions must either be made or defines wholly dropped when adding configure-time commands like C preprocessing, both of which are wrong.

Solution:
Switch the logic to a more verbose if()-style that guarantees defines that can be known at configure time are resolved.

LIST OF MODIFIED FILES:
M CMakeLists.txt
M cmake/c_preproc.cmake

RELEASE NOTE: CMake build no longer uses generator expressions in defines

[amstokely]

These changes look good to me. My only suggestion is to use target_compile_definitions where possible and to group related compile definitions into a list before passing them to add_compile_definitions or target_compile_definitions. However, I see that you've already implemented this approach in PR 2053

[amstokely]

All of these changes look good to me.