[CMake] Building the same library with different defines

[Patrick J. Franz -- ML]

I'm a fairly new CMake user, so I apologize in advance if this question 
is answered trivially elsewhere. I searched through the docs and Google 
already to no avail.

In a project I am working on, I need to build our Win32 libraries in 
both cdecl and stdcall variants. The calling conventions can be 
controlled through a -D variable, but I am having a hard time getting 
CMake to modify definitions between builds of the same set of source files.

I have tried 2 techniques:

1) Use SET_SOURCE_FILES_PROPERTIES() to change the -D settings between 
builds.

# cdecl (default) build
ADD_LIBRARY(my_lib SHARED src/lib.c)

SET_SOURCE_FILES_PROPERTIES(src/lib.c COMPILE_FLAGS -DTURN_ON_STDCALL)
ADD_LIBRARY(my_lib_stdcall SHARED src/lib.c)

On my machine, SET_SOURCE_FILE_PROPERTIES() has the effect of setting 
TURN_ON_STDCALL for every build that uses src/lib.c.


2) Use ADD_DEFINITIONS() and REMOVE_DEFINITIONS().

REMOVE_DEFINITIONS(-DTURN_ON_STDCALL)
ADD_LIBRARY(my_lib SHARED src/lib.c)

ADD_DEFINITIONS(-DTURN_ON_STDCALL)
ADD_LIBRARY(my_lib_stdcall SHARED src/lib.c)

The effect of this was no different then when I used 
SET_SOURCE_FILES_PROPERTIES().

If I had to guess, I would suppose that lib.obj is really only being 
built once and simply linked twice to create the two libraries I want. 
What I really need is to force lib.c to be built (and linked) twice with 
the different -D options. Any suggestions on how I might do this? (It 
could also be that I'm doing this the hard way and there is a more 
idiomatic way to create these two libraries that doesn't involve CMake. 
I'll take those suggestions too :)

Thanks in advance,
Patrick