poppler,poppler-devel: use MP libc++ on legacy

[erikbs]

Fixes: https://trac.macports.org/ticket/69455

Description

Fix liking on older Mac OS X versions by linking against MacPorts libc++ on platforms that use the MacPorts legacy-support library

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.9.5 13F1911 x86_64
Xcode 6.2 6C131e

Verification

Have you

• followed our Commit Message Guidelines?
• 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 --nitpick?
• 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:
@mascguy for port poppler-devel, poppler.
@dbevans for port poppler-devel, poppler.

[ryandesign]

Don't you also need to add legacysupport.newest_darwin_requires_legacy 18?

[erikbs]

Yes, I think you are right. The default value seems to be 16, and 19 is the earliest version that provides std::filesystem::path. I pushed an update.