As recommended I downloaded the latest version of GCC from http://winlibs.com/
Then I added that GCC compiler to Codelite.. Menu > Settings > Build Settings > Press the + icon to add a compiler and navigate to the bin directory of the MinGW GCC [e.g: C:\mingw64\bin] then give it a name, e.g: GCC 10.2.0
To tell Codelite to tell the GCC compiler to compile C programs against the (latest) C18 standard...
The option to specify C18 is not listed in Menu > Workspace > Open Active Project Settings > Compiler> C Compiler Options.
You can tell the GCC compiler in Codelite to use the C18 standard for a specific Project by going to Menu > Workspace > Open Active Project Settings > Compiler> C Compiler Options then click on the little ellipsis button then type in -std=c18 in the "Enter other options" textbox.
Alternatively you can add this to the list of "C Compiler Options"…
Menu > Settings > Build Settings > Compiler Options > New.
Then for "Switch" type: -std=c18 and for "Help" type: Enable C18 features.
As explained in the Language Standards Supported by GCC and from testing I note that to enforce the gcc compiler to use the C18 standard you can specify -std=c11 OR -std=c17 OR -std=c18 as a parameter to gcc.exe