config.h was removed in 2.6.19

linux.h appears in
modules/drbd/drbd/drbd_fs.c
modules/drbd/drbd/drbd_main.c
modules/drbd/drbd/drbd_proc.c
modules/drbd/drbd/drbd_receiver.c
modules/drbd/drbd/drbd_req.c
modules/drbd/drbd/drbd_worker.c

However once that hurdle is passed, the next one appears
/usr/src/modules/drbd/drbd/drbd_main.c:52:35: error: linux/devfs_fs_kernel.h: No such file or directory

Which has also been removed (since 2.6.17), it only appears in modules/drbd/drbd/drbd_main.c so I removed the line.

Next issue
if (drbd_ee_cache && kmem_cache_destroy(drbd_ee_cache))
                printk(KERN_ERR DEVICE_NAME
                       ": kmem_cache_destroy(drbd_ee_cache) FAILED\n");
Results in
/usr/src/modules/drbd/drbd/drbd_main.c:2023: error: void value not ignored as it ought to be

Also mentions that kmem_cache_t is deprecated

gcc (GCC) 4.1.2 20060928 (prerelease) (Ubuntu 4.1.1-13ubuntu5)wait...
Copyright (C) 2006 Free Software Foundation, Inc. build KVERS=2.6.20.3-custom-070319 KSRC=/usr/src/linux-headers-2.6.20.3-custom-070319 KDREV=2.6.20.3-customThis is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

GNU Make 3.81
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

This program built for i486-pc-linux-gnu

dpatch deapply-all
rm -rf patch-stamp patch-stampT debian/patched
dh_clean
/usr/bin/make -C drbd clean
make[1]: Entering directory `/usr/src/modules/drbd/drbd'
rm -rf .tmp_versions
rm -f *.[oas] *.ko .*.cmd .*.d .*.tmp *.mod.c .*.flags .depend .kernel*
make[1]: Leaving directory `/usr/src/modules/drbd/drbd'
/usr/bin/make -f debian/rules kdist_clean kdist_config binary-modules
make[1]: Entering directory `/usr/src/modules/drbd'
dpatch deapply-all
rm -rf patch-stamp patch-stampT debian/patched
dh_clean
/usr/bin/make -C drbd clean
make[2]: Entering directory `/usr/src/modules/drbd/drbd'
rm -rf .tmp_versions
rm -f *.[oas] *.ko .*.cmd .*.d .*.tmp *.mod.c .*.flags .depend .kernel*
make[2]: Leaving directory `/usr/src/modules/drbd/drbd'
/usr/bin/gcc-4.1
for templ in /usr/src/modules/drbd/debian/drbd8-module-_KVERS_.postinst /usr/src/modules/drbd/debian/drbd8-module-_KVERS_.postinst.backup /usr/src/modules/dr
bd/debian/drbd8-module-_KVERS_.postinst.modules.in; do \
    cp $templ `echo $templ | sed -e 's/_KVERS_/2.6.20.3-custom-070319/g'` ; \
  done
for templ in `ls debian/*.modules.in` ; do \
    test -e ${templ%.modules.in}.backup || cp ${templ%.modules.in} ${templ%.modules.in}.backup 2>/dev/null || true; \
    sed -e 's/##KVERS##/2.6.20.3-custom-070319/g ;s/#KVERS#/2.6.20.3-custom-070319/g ; s/_KVERS_/2.6.20.3-custom-070319/g ; s/##KDREV##/2.6.20.3-custom-07031
9-10.00.Custom/g ; s/#KDREV#/2.6.20.3-custom-070319-10.00.Custom/g ; s/_KDREV_/2.6.20.3-custom-070319-10.00.Custom/g ' < $templ > ${templ%.modules.in}; \
  done
dh_testdir
dh_testroot
dh_clean -k
/usr/bin/make -C drbd KERNEL_SOURCES=/usr/src/linux-headers-2.6.20.3-custom-070319 MODVERSIONS=detect KERNEL=linux-2.6.20.3-custom-070319 KDIR=/usr/src/linux
-headers-...

I'm also having this problem with drbd0.7 and the xen 2.6.19 kernel on i386. This is a big problem for those of us upgrading our xen/drbd clusters.

symlinking autoconf.h to config.h in /usr/src/linux/include/linux/ seems to fix the initial problem for me, but then there are more build errors in 0.7.

i had the same problem (void value not ignored) with feisty and drbd 7.21 and fixed it with the following patch:
http://lists.linbit.com/pipermail/drbd-user/2006-November/005918.html

void drbd_destroy_mempools(void)
{
        if (drbd_request_mempool)
                mempool_destroy(drbd_request_mempool);
        if (drbd_ee_cache)
                kmem_cache_destroy(drbd_ee_cache);
        /* Above code changed from the following function for compatibility
        / with mm3 patches to kernel 2.6.18:
        /
        / if (drbd_ee_cache) && kmem_cache_destroy(drbd_ee_cache))
        / printk(KERN_ERR DEVICE_NAME
        / ": kmem_cache_destroy(drbd_ee_cache) FAILED\n");
        */
        if (drbd_request_cache)
                kmem_cache_destroy(drbd_request_cache);

        /* Above code changed from the following function for compatability
        / with the mm3 patches to kernel 2.6.18:
        /
        / if (drbd_request_cache && kmem_cache_destroy(drbd_request_cache))
        / printk(KERN_ERR DEVICE_NAME
        */

        // FIXME what can we do if we fail to destroy them?

        drbd_request_mempool = NULL;
        drbd_ee_cache = NULL;
        drbd_request_cache = NULL;

        return;
}

This will be fixed for hardy.