Simplify autotools usage

Unify .xml and .desktop generation rules, replace @VAR@ reference with
$(VAR), don't hard-code PACKAGE_VERSION and PACKAGE_NAME in engine.c,
and install default.xml under $(pkgdatadir) instead of
$(pkgdatadir)/setup, since it is also read by the engine.

BUG=

Review URL: https://codereview.appspot.com/7674043