$(filter PATTERN...,TEXT)

$(filter PATTERN...,TEXT)

Returns all whitespace-separated words in TEXT that *do* match any of the
PATTERN words, removing any words that *do not* match. The patterns are written
using %, just like the patterns used in the patsubst function above.
The filter function can be used to separate out different types of strings (such
as file names) in a variable. For example:
sources := foo.c bar.c baz.s ugh.h
foo: $(sources)
cc $(filter %.c %.s,$(sources)) -o foo
says that foo depends of foo.c, bar.c, baz.s and ugh.h but only foo.c,
bar.c and baz.s should be specified in the command to the compiler.