[CMake] How do I generate a preprocessed file?

[Piotr Wyderski]

Hello,

my problem is as follow. I have a project composed of
four directories: base, io, rt and vm. The first three of them
are being built in a normal way, but the last one has some
fancy requirements. Namely, in the directory vm I want to
generate a file called inc.cpp, which is composed of all
the header files from base, io and rt, preprocessed
by the C++ compiler. So, in vm/CMakeLists.txt I specify:

ADD_DEFINITIONS(-DCONFIG_BUILDING_VM)
ADD_LIBRARY(vm SHARED inc.cpp)

Next, I indicate that inc.cpp is generated:

SET_SOURCE_FILES_PROPERTIES(${CMAKE_CURRENT_BINARY_DIR}/inc.cpp
PROPERTIES GENERATED 1)

Then I create a list of all the header files of interest:

FILE(
    GLOB
    _runtime_header_list
    RELATIVE ${CMAKE_CURRENT_SOURCE_DIR}
    ${PROJECT_SOURCE_DIR}/base/*.h
    ${PROJECT_SOURCE_DIR}/io/*.h
    ${PROJECT_SOURCE_DIR}/rt/*.h)

And specify a command to generate the file:

ADD_CUSTOM_COMMAND(

   OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/inc.cpp
   COMMAND YYY >inc.cpp
   DEPENDS ${_runtime_header_list}
)

but what shoud YYY be in order to achieve the following goal:

1. all the entries from _runtime_header_list are enumerated
and a single temporary file t1.cpp is created as follows:

   #include "entry_1"
   #include "entry_2"
   ...
   #include "entry_n"

2. t1 is sent to the C++ compiler (with all its options,
definitions and include paths set as for a normal build),
but for preprocessing stage only (i.e. g++ -E). The resulting
C++ file is my inc.cpp.

How can I achieve that goal using CMake? Of course
it doesn't need to be done the way I specified above,
as I am pretty new to CMake. A functional equivalent
is all what I need, in particular I don't need t1.cpp.

Best regards
Piotr Wyderski