There is a C language version in the build settings that let's you set the language version to C11.
You can just turn off that particular setting with a build flag: -Wno-implicit-function-declaration
Many open source packages won't build otherwise. Just be careful with it. While some packages require it, other packages may check for the lack of certain functions (to determine which version of a dependency to use) and this would break those packages. I have a project right now that has some of both.
I wouldn't recommend using it on your own code. It is better to properly declare your functions.