The "make clean" rule added by gnome-code-coverage.m4 was not
conditionalized (making it spew errors on "make clean" when built
without coverage support, since $(LCOV) expanded to ""), and the other
rules were conditionalized incorrectly (testing "ifdef
CODE_COVERAGE_ENABLED", when that variable was always set, just
sometimes to "no"). Fix both problems.

https://bugzilla.gnome.org/show_bug.cgi?id=699943