FindRuby: Restore compatibility variable RUBY_INCLUDE_PATH
Since !4463 (merged), the result variables named
with the old RUBY_
prefix are provided by compatibility code that maps
from the new Ruby_
prefix variables. There is no Ruby_INCLUDE_PATH
variable, so do not try to map it to RUBY_INCLUDE_PATH
. The latter is
provided by dedicated compatibility code left from before that
transition.
Fixes: #21064 (closed)
Backport: release