Created attachment 379804 [details]
Long story short, /usr/bin/libtool uses whatever random CC/CXX was used when libtool was last installed. So, using system-wide libtool means mixing CC/CXX used when building the package with CC/CXX used when building libtool, with the latter likely no longer installed.
Solution: only possible I know is to use autotools macros to have proper local libtool suited for currently used CC/CXX. Or not use libtool at all.
Well, as we now continue to ship slot :5.1, I'll look into using configure.in for this one, too (notice how lua-5.2 already generates it's own libtool files).
lua-5.2 also runs /usr/bin/libtool