default option value in the ${name-of-option} syntax support
From the doc:
https://docs.kde.org/trunk5/en/kdesrc-build/kdesrc-build/kdesrc-buildrc.html#kdesrc-buildrc-layout
Processing of option values
In general, the entire line contents after the option-name is used as the option-value.
One modification that kdesrc-build performs is that a sequence ${name-of-option} is replaced with the value of that option from the global configuration. This allows you to reference the value of existing options, including options already set by kdesrc-build.
This resembles Shell Parameter Expansion.
That expansion has extended form when Bash tests for a parameter that is unset or null:
${parameter:-word}
If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.
Do you think it worth to add the same support for the options?