build: don't pass LDFLAGS from environment to curl

After ea5929db ('build: fix OpenSSL
linking problems on FreeBSD') we set CFLAGS explicitly (possibly to an
empty value) when invoking a configure script for curl. When this
parameter is set the script does not use a value of environment variable
CFLAGS.

Before this commit LDFLAGS environment variable can affect build of curl
submodule. This can lead to a problem when a user or a tool set CFLAGS
and LDFLAGS both and some linker flag assumes that some compilation flag
is present. Here we set empty LDFLAGS explicitly to avoid using of the
environment variable.

A distributive build tool such as rpmbuild or emerge usually sets CFLAGS
and LDFLAGS. The problem with incompatible compiler / linker options has
been reveal under rpmbuild on CentOS 8 with hardened build enabled
(which is so when backtraces are disabled).

It is not clear whether we should follow environment variables or values
determined by CMake for CFLAGS, CPPFLAGS and LDFLAGS when building a
submodule (such as luajit and curl). Let's decide about this later.

Part of #4543.

Reviewed-by: Alexander V. Tikhonov <avtikhon@tarantool.org>
Reviewed-by: Igor Munkin <imun@tarantool.org>