You can override the variables manually, e.g. replacing some flags in
the linker command, but you can't change the value of the variables in
sharp braces. Usually you don't have to change these rules, only in rare
cases. You should only do this if you know what you are doing and there
is no other way.
There are a couple dozen different less-then-greater-then tags that you
can put in build rules. From examining the source code the following
style names exist: