build: Override systemd unit directory for distcheck

distcheck tests that all the files are installed under $prefix. That
doesn't work with the systemd unit directory since the path comes from
pkg-config. Override the setting to be under $prefix in that case.

Closes: #372
Approved by: cgwalters

Makefile-boot.am

@@ -38,6 +38,9 @@ endif
 if BUILDOPT_SYSTEMD
 systemdsystemunit_DATA = src/boot/ostree-prepare-root.service \
 	src/boot/ostree-remount.service
+
+# Allow the distcheck install under $prefix test to pass
+AM_DISTCHECK_CONFIGURE_FLAGS += --with-systemdsystemunitdir='$${libdir}/systemd/system'
 endif
 
 if !BUILDOPT_BUILTIN_GRUB2_MKCONFIG