CMakeLists.txt

project(test-cmake CXX)
cmake_minimum_required(VERSION 3.9)

# 
set(header_in ${CMAKE_CURRENT_BINARY_DIR}/header.in)
file(WRITE ${header_in}
"
#ifndef MY_HEADER
#define MY_HEADER
  int my_x = 42;
#endif
"
)

set(header_file ${CMAKE_CURRENT_BINARY_DIR}/header.h)
add_custom_command(OUTPUT ${header_file}
    COMMAND cp ${header_in} ${header_file}
    DEPENDS ${header_in}
)
set_source_files_properties(${header_file} PROPERTIES GENERATED TRUE)
add_custom_target(my_headers DEPENDS ${header_file})

include_directories(${CMAKE_CURRENT_BINARY_DIR})

add_library(my_lib STATIC libfile.cpp ${header_file})
add_dependencies(my_lib my_headers)

header.in

#ifndef HEADER_H
#define HEADER_H

int my_x = 42;

#endif // HEADER_H

libfile.cpp

#include "header.h"

int foo() {
 return my_x; 
}