Install location for 4ti2 headers

[mkoeppe]

groebner, util, zsolve should perhaps appear as subdirectories of ${prefix}/4ti2.
Check what distributions do.

[jengelh]

4ti2 headers seem to be in the right place as of 1.6.9. Just the programlets/scriptlets like groebner etc. should either be prefixed (note how /usr/bin/4ti2gmp already has one!), or be moved to ${pkglibexecdir} aka /usr/libexec/4ti2. /usr/4ti2 is generally not an accepted path (/usr/X11R6 was the last software that was “allowed” such a special position, and even that is years past).