If I understand this, you're looking for a way to tell Sublime to build a specific file regardless of which file has focus/is open at the time?
The Build Systems pages on st.info can probably help:
sublimetext.info/docs/en/referen ... stems.html
From there you can see this snippet:
"cmd": "python", "-u", "$file"],
"file_regex": "^ ]*File \"(...*?)\", line ([0-9]*)",
Simply replace "$file" on the cmd line with an absolute path to the main file of your project and save it as a .sublime-build file in your User directory. This should give you a project-specific build system which you can select from the menu. This approach isn't very flexible and you'll need to edit it or create a new .sublime-build file for each project. If your projects share a common naming scheme and folder structure, you might try using either "$file_path" or "$project_path" variables to create a more robust build file. This can be handy if you move between environments like Windows and Linux, or have multiple workstations that may not be configured the same way.