Syntax for help Ifneq ($ (Filter AAA BBB CCC, $ (Var)),)

Filter is the meaning of filtering, its prototype is: $ (filter pattern...,text),
Meaning: Filter out all words in the string "TEXT" that do not conform to the pattern, and keep all words that conform to this pattern to return the value.
Combined with the front ifneq can be understood in this way: Ifneq (A,), where a=$ (filter AAA BBB CCC, $ (Var))
Variable var if the triple-C BBB CCC one or more of the three, A is not empty, then Ifneq (A,) is true, that is, the original statement is true.

This is makefile's grammar, recommended "write with me Makefile"