Swap from AM_CPPFLAGS to ostree_prepare_root_CPPFLAGS when compiling
ostree-prepare-root statically. This fixes a problem when you have
systemd and libmount, but only ostree_prepare_root_CPPFLAGS includes
-DHAVE_SYSTEMD_AND_LIBMOUNT=1.
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Closes: #1670
Approved by: jlebon