Compile C files in C++ project which do not use precompiled header?
In the Solution Explorer window right click on the *.c file(s) and select Properties. Go to C / C++ -> Precompiled Headers and set the Precompiled Header option to Not Using Precompiled Headers.
Also, unless you actually need precompiled headers, I'd say turn it off project-wide.
Another option would be to compile your C files as C++ and keep using the precompiled headers. To do that, right click on the project name (or each .c file name), and set C / C++ -> Advanced -> Compiles As to Compile as C++ code.