Added C/C++ file to Eclipse Does not Compile - c++

Question

This has to be some kind of newbie question, but I have not been able to find any explaination.
I am running Ubuntu 18, and need to work with some C/C++ files. I've been using TI's CCS which is eclipse based on Windows for years.
I downloaded the Eclipse installer and ran it setting up for C/C++ developers.
https://www.eclipse.org/downloads/packages/installer
I created a new project. There were several different (unexplained) options such as CDT, MESON, MakeFile, ... I have tried several.
Creating a HelloWorld source file, it compiles and runs fine.
#include <stdio.h>
int main() {
puts("Hello World");
return 0;
}
Okay, so far...
Now I add a new source file. Called "OtherFile.c"
#include <stdio.h>
void OtherFunction() {
puts("Other Hello");
}
And of course, modify the original:
#include <stdio.h>
extern "C" void OtherFunction();
int main() {
puts("Hello World");
OtherFunction();
return 0;
}
When I try to build, it will not compile the new file. And (as expected) it tell me that "OtherFunction" is unresolved.
I have tried multiple project types (CDT, Meson, Makefile) even though there is no explanation of the differences. The newer file will not be compiled.
I tried changing the file extension from c to cpp and back. The newer file will not be compiled.
The TI version of CCS using Eclipse will include a source file when it's in the folder. However, in this environment, I cannot convince Eclipse to compile any other file than the one that was originally created by the new C/C++ project step.
And just as annoying is the fact that I can't right click either file and "Build Selected File". The menu option doesn't even appear.
This did not work for me:
eclipse c/c++ CDT build just one file
Can someone advice how to convince Eclipse to compile additional files?
TIA.
EDIT:
I can't upload here, so I just created something on GitHub.
These are two of the samples where I added a second file, and it ignores it.
https://github.com/scotty2541/EclipseExample
In all the other things I've done in Eclipse, it simply uses a default "recipe" like make does to compile the file.
If there is some way to manually tell Eclipse about it, that isn't explained anywhere I've been able to find. And seems to defeat the purpose of the IDE's behavior.

Answer 1

I was able to get it to behave as expected: By choosing a CDT managed build system, when adding a file to the project, it compiles it using the default recipe
Then, there is a setting which causes it to run the "builder" after a clean.
When I added the file as described originally, I also had to do a "clean" in order for the environment to include the additional file.