lowdown: Predefine linker args for darwin <= 14

[snowflake]

mktemp(1) on darwin <= 14 requires a template, which configure is not providing, so use predefined values in the Portfile.

Closes https://trac.macports.org/ticket/70651
Closes https://trac.macports.org/ticket/71204

Description

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.11.6 15G22010 x86_64
Xcode 8.2.1 8C1002

Verification

Have you

• followed our Commit Message Guidelines?
• [ x] squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@judaew for port lowdown.