Changing the make invocation in dkms.conf to:
"make module KERNDIR=/lib/modules/$kernelver IGNORE_XEN_PRESENCE=1 IGNORE_CC_MISMATCH=1 SYSSRC="/lib/modules/$kernelver/source" SYSOUT=$kernel_source_dir"
fixes the problem.
DKMS sets $kernel_source_dir to the build dir and has no specific variable for the source dir.
Changing the make invocation in dkms.conf to:
"make module KERNDIR= /lib/modules/ $kernelver IGNORE_ XEN_PRESENCE= 1 IGNORE_ CC_MISMATCH= 1 SYSSRC= "/lib/modules/ $kernelver/ source" SYSOUT= $kernel_ source_ dir"
fixes the problem.
DKMS sets $kernel_source_dir to the build dir and has no specific variable for the source dir.