conditional compilation on wince

While designing BSP, it's very common to enable or disable some features based on our requirements. Subsequently, we may need to change our code according to the feature's availability.
On a wince system, the status of a feature's availability is controlled via batch files, either cesysgen.bat (controls standard ce modules and functions) or {platform_name}.bat (controls platform specific settings). The status of a feature is set through environment variable in the batch file. The build system will build the image according to environment variables. For instance, the following line in a batch file instruct not to support SDMMC boot.

set BSP_NOSDMMC_BOOT=1

It may be desired to change our code based on the aforementioned setting. But the problem is the compiler (to be specific, preprocessor) can't see the value of BSP_NOSDMMC_BOOT environment variable. So, in order to pass the value to compiler, we need the help of build system. Just like gnu make, the wince build system can see the environment variable and define a macro for the compiler.
The code snippet below is extracted from a sources file, and shows how to define macro according to the BSP_NOSDMMC_BOOT's value.

!IF "$(BSP_NOSDMMC_BOOT)"=="0"
CDEFINES = $(CDEFINES) -DBSP_NOSDMMC_BOOT
!ENDIF

Then, in our code, we can use the BSP_NOSDMMC_BOOT macro to do conditional compilation.
And if we want to disable the whole module based on the environment variable's value, we can define SKIPBUILD in sources file, like this:

!if "$(BSP_NOSDMMC_BOOT)"=="1"
SKIPBUILD = 1
!endif