cmake: disable -fvar-tracking-assignments for config.cc

cmake: disable -fvar-tracking-assignments for config.cc

there are too many variables involved in config.cc, but
-fvar-tracking-assignments is enabled by default, when the compiler
finds that the number of variables exceeds the limit
"max-vartrack-size", it gives up, and tries again disabling this
option.

this change avoids the recompilation of config.cc and also
silences the warning of

src/common/config.cc:77:1: note: variable tracking size limit exceeded
with -fvar-tracking-assignments, retrying without
 md_config_t::md_config_t(bool is_daemon)
 ^

Signed-off-by: Kefu Chai <kchai@redhat.com>