To allow additional flags, set CGO_CFLAGS_ALLOW to a regular expression
matching the new flags. To disallow flags that would otherwise be allowed,
set CGO_CFLAGS_DISALLOW to a regular expression matching arguments
that must be disallowed. In both cases the regular expression must match
a full argument: to allow -mfoo=bar, use CGO_CFLAGS_ALLOW='-mfoo.*',
not just CGO_CFLAGS_ALLOW='-mfoo'. Similarly named variables control