Build: allow -Werror to be disabled even on CI branches (#1471)
Build: allow -Werror to be disabled even on CI branches (#1471)

Slight reworking of -Werror logic to let the default behavior be
changed by setting a cmake variable. In particular, this allows us to
make a CI build disable -Werror when we need to.

Previously, having the "OSL_CI" env variable set trumped all else and
forced -Werror. Now it just contributes to the default behavior, but
setting `-DSTOP_ON_WARNING=OFF` and cmake config time can still
disable it.

Signed-off-by: Larry Gritz <f0a1ce6f80112d72f7276457fd261d671b827567@larrygritz.com>
brechtvl/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,imageworks/OpenShadingLanguage,lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage,aconty/OpenShadingLanguage,aconty/OpenShadingLanguage,brechtvl/OpenShadingLanguage,brechtvl/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage