Set default value for EXCLUDE_DEPRECATED_BEFORE_AND_AT

ecm_generate_export_header fails to warn if an undefined variable
(and thus no value) is passed to its EXCLUDE_DEPRECATED_BEFORE_AND_AT flag.

Things work by chance, as "0" is also the internal default, but better
have the variable always defined & documented, like elsewhere.

GIT_SILENT

(cherry picked from commit bb3b5601)