makefile: prevent hypothetical double expansion

Given the following makefile:

    X = $(shell echo '$$HOME')

    example:
        @echo 'single-quoted: $(X)'
        @echo "double-quoted: $(X)"

Running `make` will output something similar to the following:

    single-quoted: $HOME
    double-quoted: /Users/dc

Though double expansion will almost certainly never occur in practice,
there's no downside to correctness in this case.