CMake build no longer uses generator expressions in defines (#2056)

CMake build no longer uses generator expressions in defines (#2056)

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