After a multiline argument ending in a non-empty line e.g.
if(TEST)
set(VAR "
...")
unset(VAR)
endif()
we previously failed to indent following lines like the "unset". Use
cmake-line-starts-inside-string to keep walking back through multiline
arguments until we find an indented line.
Suggested-by: Christopher Sean Morrison <brlcad@mac.com>