Export libcurl's symbols only when they are provided by tarantool
itself: when the library is linked statically into the tarantool's
executable. There is no much sense to export the symbols when we link
against the library dynamically.

Regarding motivation of the change. Since 2.6.0-36-g29ec62891 ('Ensure
all curl symbols are exported') the curl_multi_poll() function is
exported from the tarantool executable. It leads to a failure in
Homebrew's build, because there we link (dynamically) with a system
libcurl. On Mac OS 10.15 it is libcurl 7.64.1, while the function
appears since libcurl 7.66.0. So a linker reports the undefined symbol:
`curl_multi_poll`.

Now the symbols are not exported at dynamic linking with libcurl, so the
linker is happy.

This commit relaxes bounds for dynamic linking, but an attempt to link
with libcurl older than 7.66.0 statically still leads to a linking
failure. The box-tap/gh-5223-curl-exports.test.lua test still fails when
tarantool is linked (dynamically) against an old libcurl.

It looks as the good compromise. When libcurl functionality is provided
by tarantool itself, *all* functions listed in the test are present
(otherwise a linker will complain). But tarantool does not enforce a
newer libcurl version, when it just *uses* this functionality and don't
provide it for modules and stored procedured. It is not tarantool's
responsibility in the case.

We possibly should skip the box-tap/gh-5223-curl-exports.test.lua test
when tarantool is built against libcurl dynamically or revisit the
described approach. I'll leave it as possible follow up activity.

Fixes #5542