The install process of traditional extended library (=non gem) uses extconf.rb for creating Makefile and install the library by (n)make install.
However it does not work correctly because of FileUtil.mkdir met bad directory name as 'c:c:/...'.
This is caused by DESTDIR and prefix combination.
When one does not specify DESTDIR, rbconfig.rb create it as 'DESTDIR = TOPDIR && TOPDIR[/\A[a-z]:/i] || '' unless defined? DESTDIR' #=> 'C:' or 'D:'.
By the other hand CONFIG['prefix'] is (TOPDIR || DESTDIR + '/usr') so the prefixdir also contains 'c:' or 'd:' ...
The definition for install directories in mkmf.rb is defined as both DESTDIR and CONFIG['prefix'] directory for example '$(DESTDIR)$(vendorarchdir)$(target_prefix)'.
It causes the double drive letter name 'c:c:...'.
While there are some workarounds as using setup.rb or gem, it's harmful for the testing phase of extended libraries.
This issue was solved with changeset r44019.
Akio, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.