Codechange: split building into a library and executable

src/os/unix/CMakeLists.txt

@@ -1,6 +1,5 @@
 add_files(
     crashlog_unix.cpp
-    unix_main.cpp
     CONDITION UNIX AND NOT APPLE AND NOT OPTION_OS2
 )
 
@@ -13,3 +12,7 @@ add_files(
     font_unix.cpp
     CONDITION Fontconfig_FOUND
 )
+
+if(UNIX AND NOT APPLE AND NOT OPTION_OS2)
+    target_sources(openttd PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/unix_main.cpp)
+endif()