gmt6: update to 6.0.0rc5

[seisman]

Description

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.14.6 18G95
Xcode 10.3 10G8

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?
• 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]

Notifying maintainers:
@tenomoto for port gmt5.

[macportsbot]

Travis Build #8865 Passed.

Lint results

--->  Verifying Portfile for gmt5
--->  0 errors and 0 warnings found.

Port gmt5 success on xcode10.3. Log
Port gmt6 success on xcode10.3. Log
Port gmt5 success on xcode9.4. Log
Port gmt6 success on xcode9.4. Log
Port gmt5 success on xcode8.3. Log
Port gmt6 success on xcode8.3. Log
Port gmt5 success on xcode7.3. Log
Port gmt6 success on xcode7.3. Log

[seisman]

Can someone approve/merge this?