it still seems odd that in case of --new-style only include_prefix can be set, but others like lib_prefix are ignored.
Well, the whole idea of --new-style is to collect everything under a common prefix, so maybe it's not so odd after all.
i'll commit a patch setting include_prefix=$(prefix)/include/pike and passing it to install.pike in src/Makefile.in.
Hm, I just realized a complication with this. While it is true that the default value in install.pike is always combine_path(prefix,"include","pike"), the value of the "prefix" is not necessarily the same as $(prefix). In the case of a --new-style install (which is default), the prefix is modified by the code before include_prefix is computed. So hardcoding it in the Makefile using the make variable $(prefix) would not give the same result. :-/