seqan3: new port

[rrahn]

Description

SeqAn3 - The modern C++ library for sequence analysis.
This is the new version of the SeqAn library.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.15.
Xcode 12.0
macports-gcc-10

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?
• 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?

[macportsbot]

Travis Build #14596 Passed.

Lint results

--->  Verifying Portfile for seqan3
--->  0 errors and 0 warnings found.

Port seqan3 success on xcode10.3. Log
Port seqan3 success on xcode9.4. Log
Port seqan3 success on xcode8.3. Log