In commit 7669695a (CheckIncludeFiles: extend to allow a LANGUAGE
argument, 2017-11-08) the file name used for the check was accidentally
left as `${var}.c`, where `var` is not a variable we define.  It was
passing tests by accidentally using just `.c` as the file name, but can
break in real projects that may leave `var` defined to something else.
The reference was taken from similar code in CheckTypeSize where the
variable names are different.  Fix our reference to be `${VARIABLE}`.