build: Fix failing to install unit file on distcheck

During distcheck we fail to install the systemd user session unit file
because it's trying to install to the place returned by pkg-config. We
could add a --with-systemduserunitdir= and set it to a proper install
place. This is done in some other programs like systemd. However there's
not much to test for the unit files installation during distcheck so
just pass --disable-systemd that will do the right thing.

Fix the error below:

 /usr/bin/install -c -m 644 e18.service '/usr/lib/systemd/user'
/usr/bin/install: cannot remove ‘/usr/lib/systemd/user/e18.service’: Permission denied
make[4]: *** [install-unitsDATA] Error 1
make[4]: Leaving directory `/home/lucas/p/e/enlightenment/enlightenment-0.17.99.17181/_build/data/units'
make[3]: *** [install-am] Error 2
make[3]: Leaving directory `/home/lucas/p/e/enlightenment/enlightenment-0.17.99.17181/_build/data/units'
make[2]: *** [install-recursive] Error 1
make[2]: Leaving directory `/home/lucas/p/e/enlightenment/enlightenment-0.17.99.17181/_build/data'
make[1]: *** [install-recursive] Error 1
make[1]: Leaving directory `/home/lucas/p/e/enlightenment/enlightenment-0.17.99.17181/_build'
make: *** [distcheck] Error 1

Makefile.am

@@ -66,6 +66,8 @@ po/Rules-quot
 
 endif
 
+DISTCHECK_CONFIGURE_FLAGS = --disable-systemd
+
 EXTRA_DIST = config.rpath  README AUTHORS COPYING \
              enlightenment.spec.in enlightenment.spec enlightenment.pc