Remove -Werror from all the default cflags.

I've recently been coming round in general to the idea that -Werror is
fine for developers and centralised binary builds, but has too many
unanticipated failure modes in the field (with everyone's different
versions of compilers, headers etc) to leave turned on for the 'just
download and build' source tarball that's supposed to work everywhere.
On main, I've already made the change to hide it behind a cmake
'strict' setting.

In particular, I've just done pre-release build tests with various
versions of GTK, which reminded me that the GTK 2 installation on
Ubuntu 20.04 fails to build at -Werror, because GTK's own header files
have a warning-generating inconsistency. (glib/gtypes.h declares
GTimeVal as deprecated, and then gtk/gtktooltips.h uses it anyway.)
Clearly this is the kind of thing that ought not to break the build of
a client application!