Besides that is generally good practice to avoid macro redefinitions
(and symbol redeclarations), and we're doing this on POSIX platforms
anyway, there is a particular issue regarding phpize builds, where
config.w32.h actually includes config.pickle.h. The latter overrides
some macro definitions (e.g. `PHP_BUILD_SYSTEM`) to define the proper
values, but if config.w32.h is included multiple times, different macro
definitions eventually raise C4005 compiler warnings[1], which break
builds with `/WX /W1` enabled.
[1] <https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4005>