The -x option for gcc lets you specify the language of all input files following it:
$ gcc -x c your-file-name.cpp
If you only want to special-case that one file, you can use -x none to shut off the special treatment:
$ gcc -x c your-filename.cpp -x none other-file-name.cpp
(your-filename.cpp will be compiled as C, while other-file-name.cpp will use the extension and compile as C++)