gcc: -Dvar=value not working as expected - gcc

Question

I am trying to compile a library that has these lines:
#if AE_OS==AE_WINDOWS
#include windows.h
//stuff
#elif AE_OS==AE_POSIX
//other stuff
#endif
When I use cpp -DAE_OS=AE_POSIX I get
cpp/src/ap.cpp:63:21: fatal error: windows.h: No such file or directory
#include <windows.h>
^
Adding a space after the -D does not work, nor does putting AE_OS=AE_POSIX in single or double quotes, putting only AE_POSIX in single or double quotes, and trying all of these combinations with the quotes escaped.
Putting AE_POSIX in escaped single quotes at least does something different, but it is apparently still not correct:
cpp/src/ap.cpp:59:5: warning: character constant too long for its type [enabled by default]
#if AE_OS==AE_WINDOWS
^
I have also tried -D"AE_OS AE_POSIX" thinking that would be the same as #define "AE_OS AE_POSIX" but apparently it ignores the quotes since it defines AE_OS as 1:
<command-line>:0:16: error: missing binary operator before token "1"
cpp/src/ap.cpp:65:7: note: in expansion of macro ‘AE_OS’
#elif AE_OS==AE_POSIX
What is the correct way to do this?

Answer 1

The preprocessor understands expressions of integer type, not strings.
AE_POSIX and AE_WINDOWS are defined as different integers:
/*
* definitions
*/
#define AE_UNKNOWN 0
#define AE_MSVC 1
#define AE_GNUC 2
#define AE_SUNC 3
#define AE_INTEL 1
#define AE_SPARC 2
#define AE_WINDOWS 1
#define AE_POSIX 2
So for POSIX do this:
cpp -DAE_OS=2