From the Makefile:

export KDIR KVER O
KDIR := $(shell echo /lib/modules/`uname -r`/build)
KVER := $(shell KDIR=$(KDIR) O=$(O) scripts/get_uts_release.sh)




get_uts_release.sh:
#!/bin/bash
{
    for x in include/linux/{utsrelease,version}.h;
    do
        for d in $KDIR $O;
        do
            test -e "$d/$x" || continue;
            echo "#include \"$d/$x\"";
        done;
    done;
    echo "drbd_kernel_release UTS_RELEASE"
} | gcc -nostdinc -E -P - | sed -ne 's/^drbd_kernel_release "\(.*\)".*/\1/p'