CheckIncludeFiles: Fix name of source file used for check

In !1464 (merged) 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}.