build: Add lcov make targets from telepathy-glib

build: Add lcov make targets from telepathy-glib

Add lcov.mk, copied from telepathy-glib:
http://cgit.freedesktop.org/telepathy/telepathy-glib/tree/tools/lcov.am
with the tp-glib–specific bits removed.

These targets are only useful if folks is compiled with
--enable-code-coverage.

To run the test suite and generate a report, run `make lcov-check`. To just
generate a report without running the test suite, run `make lcov-report`.

The .info file and generated report are only deleted by
`make maintainer-clean`, since the developer might want to refer to them
across other cleans.

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=659610