hw/kdrive/Makefile.am gets modified to disable drivers that are not being built. The line containing VESA_SUBDIRS has a \ at the end. There is code to remove the \ if certain drivers are not being built because of use flags, but drivers can also be disabled on non-x86 platforms regardless of use flags. In the latter case, the continuation line can wind up being empty without the code to execute the \ being executed. I would suggest patching Makefile.am to combine the two lines rather than trying to conditionally remove the \ in the ebuild; I just did this on my system and successfully got a build.
Steps to Reproduce:
1. USE=kdrive emerge xorg-server (on non-x86 system)
Created attachment 133212 [details]
build log with the failure
Created attachment 133214 [details, diff]
patch to xorg-server to combine lines that are modified by the ebuild
*** Bug 184997 has been marked as a duplicate of this bug. ***
Sounds reasonable. Could you attach an ebuild patch too?
Created attachment 133483 [details, diff]
Ebuild patch as requested
Should be fixed in 188.8.131.52, please reopen if it's not.