build: Fix CFLAGS and LDFLAGS for the gcov target
build: Fix CFLAGS and LDFLAGS for the gcov target

A copy and paste thinko duplicated the CFLAGS into the LDFLAGS, so we
need to fix that. The CFLAGS should also specify the optimization level
to 0 and turn on debugging notes, in case --enable-debug and
--enable-maintainer-flags are turned off.

brauliobo/json-glib,ebassi/json-glib,brauliobo/json-glib,Distrotech/json-glib,ebassi/json-glib,robtaylor/json-glib-gvariant,GNOME/json-glib,oerdnj/json-glib,oerdnj/json-glib,oerdnj/json-glib,ebassi/json-glib,oerdnj/json-glib,Distrotech/json-glib,frida/json-glib,GNOME/json-glib,Distrotech/json-glib,frida/json-glib,robtaylor/json-glib-gvariant,brauliobo/json-glib