fix scan-build issues

The static analyzer scan-build had a few issues with analysing
parts of systemd.

gpt-auto-generator.c:
scan-build could not find blkid.h. Whether it should be blkid.h or
blkid/blkid.h seems to depend on the version used. We already use
blkid/blkid.h in udev-builtin-blkid.c so it seems safe to use that
here too.

Makefile.am:
Moved some -D's from CFLAGS to CPPFLAGS. I also simplified them a
bit and got rid of a left over DBUS_CFLAGS.

test-cgroup-mask.c/test-sched-prio.c
A variable was added to store the replaced TEST_DIR. When wrapped
in an assert_se TEST_DIR was not replaced in the logged error.
While not an issue introduced in this patch we might as well fix
it up while we are here.