Problem with Objective-C import on the build server

[janosch1 at mailbox.org]

I am working on a port called MacPassHTTP (link to PR <https://github.com/macports/macports-ports/pull/10162>). It's a plugin and therefore depends on some Objective-C headers of its parent application MacPass. On the build server, the header is not found. On my Mac it works.

MacPass installs the header into: ${prefix}/include/MacPass

In MacPassHTTP/Portfile I set the header path like this:
xcode.destroot.settings HEADER_SEARCH_PATHS=${prefix}/include/MacPass

MacPassHTTP uses this import:
#import „MPPlugin.h“

On the server, I can see that MacPass does install the header as expected:
2021-03-22T19:49:01.8946050Z DEBUG: activating file: /opt/local/include/MacPass/MPPlugin.h

But later, building MacPassHTTP fails:
2021-03-22T19:52:03.0458050Z /opt/local/var/macports/build/_Users_runner_work_macports-ports_macports-ports_ports_security_MacPassHTTP/MacPassHTTP/work/MacPass-MacPassHTTP-b445602/MacPassHTTP/MPHMacPassHTTP.h:10:9: fatal error: 'MPPlugin.h' file not found
2021-03-22T19:52:03.0459850Z #import "MPPlugin.h"
2021-03-22T19:52:03.0460610Z         ^~~~~~~~~~~~
2021-03-22T19:52:03.7206900Z 1 error generated.

Interestingly, referencing other Frameworks with FRAMEWORK_SEARCH_PATHS works also on the server. But referencing headers does not. 

The raw logs can be viewed here <https://pipelines.actions.githubusercontent.com/GKxOQnS2ECz9Gaji3FEn5hedlXhA9eesLC1FSXCsbzejZyCfqf/_apis/pipelines/1/runs/3729/signedlogcontent/4?urlExpires=2021-03-22T19:56:49.8784130Z&urlSigningMethod=HMACV1&urlSignature=UPPVfjtbrz9PGxzRL6sjUTZPJiATTTaJa/spSC83D2Q=>.

What am I doing wrong?
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