When you set the project properties for your project, the relevant properties apply to all files in the project. You can set some properties for a specific file.
Right-click the newmain.c source file and choose Properties.
Click the General node in the Categories panel and see that you can select a different compiler or other tool to build this file. You can also select a checkbox to exclude the file from the build of the currently selected project configuration.
Click the C compiler node and note that you can override the project compiler settings and other properties for this file.
Cancel the Project Properties dialog box.