CMakeLists.txt

SET( ${PROJECT_NAME}_SOURCES
  main.cpp
)

#main.x depends on main.cpp (is generated based on main.cpp)
#main.o depends on main.cpp and main.x (main.x is included in main.cpp)
#So if main.cpp changes, first run the tool to create main.x,
#then run cc to create main.o from main.cpp and main.x 
ADD_CUSTOM_COMMAND(
  WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
  OUTPUT ${PROJECT_SOURCE_DIR}/main.x
  COMMAND ./tool.sh main.cpp main.x
  DEPENDS ${PROJECT_SOURCE_DIR}/main.cpp
)

SET_PROPERTY(SOURCE ${PROJECT_SOURCE_DIR}/main.cpp
  APPEND PROPERTY OBJECT_DEPENDS ${PROJECT_SOURCE_DIR}/main.x)

ADD_EXECUTABLE(${PROJECT_NAME} ${${PROJECT_NAME}_SOURCES})
TARGET_LINK_LIBRARIES( ${PROJECT_NAME})

main.cpp

#include <cstdio>
#include "main.x"

int main()
{
	printf("%s\n", txt);
	return 0;
}

tool.sh

#!/bin/bash
echo "const char* txt = \"$1\"" >> $2