functions

$(dir /path/to/file.txt) - get path part only
$(notdir src/foo.c hacks) - result foo.c hacks.
$(suffix …)
$(basename src/foo.c src-1.0/bar hacks) - produces the result src/foo src-1.0/bar hacks.
$(addsuffix)
$(addprefix)
$(join)
$(wildcard) - execute wildcard expansion (needed to expand wildcards in variables and arguments in functions). More: The Function wildcard
$(realpath) - NOTE: file must exist!
$(abspath) - NOTE: do not require file to exist