Force -lgcov harder when running `make coverage`.

Shouldn't be needed, but I had to make this tweak to make this work again.

Makefile.am

@@ -95,7 +95,7 @@ debug:
 	$(MAKE) all CFLAGS="" AM_CPPFLAGS="-ggdb -DDEBUG"
 
 coverage:
-	$(MAKE) all CFLAGS="" AM_CPPFLAGS="-fprofile-arcs -ftest-coverage -DDEBUG" AM_LDFLAGS="-lgcov"
+	$(MAKE) all CFLAGS="" AM_CPPFLAGS="-fprofile-arcs -ftest-coverage -DDEBUG" LDFLAGS="-lgcov"
 
 .c.h:
 	@srcdir@/scripts/MakeHeader.py $<