Undocumented stuff in `make'

[Eemeli Kantola]

I discovered an undocumented feature in `make´ (version 3.79.1 on MS-DOS). There
seems to be a variable assignment operator, `?:=', which is not worth a mention
according to the `make' documentation (file `info/make.info').

This can be found out by running the following test makefile:

# makefile
VAR ?:= $(FOO)
FOO := default

all:
        @echo "$(VAR)"
# end of makefile

As a result, provided the variables `VAR' and `FOO' do not exist, the program
echoes "" on the screen, indicating that `VAR' contains a null string.