Add 'override' to all accumulating variables

[martin-g]

Without override the extra values are not appended if the variable is provided by the user, e.g. make CFLAGS="my custom flags"

I didn't add override to OBJS because I think it is not supposed to be provided by the user anyway.

[martin-g]

Here is a mini reproducer:

Makefile.demo:

FOO=foo

.PHONY:all

override FOO+=bar

all:
        echo $(FOO)

Execute without a custom value:

make -f Makefile.demo
echo foo bar
foo bar

Execute with a custom value:

make -f Makefile.mine FOO=baz
echo baz bar
baz bar

Without override in override FOO+=bar it will be:

make -f Makefile.mine FOO=baz
echo baz
baz