[MacPorts] #56107: make kchmviewer build again, respecting "our" build settings

[MacPorts]

#56107: make kchmviewer build again, respecting "our" build settings
----------------------+------------------------
 Reporter:  RJVB      |      Owner:
     Type:  defect    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:
 Keywords:  haspatch  |       Port:  kchmviewer
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 Cf. https://paste.macports.org/864f6be68dc9

 kchmviewer failed to configure because it's set up for an out-of-source
 build and the mainstream qmake5 PG fails to check if the build directory
 exists before attempting to create a file in it.

 It also fails to respect the default linker flags (configure.ldflags),
 which I've addressed together with another change to respect
 configure.optflags c.s.

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Ticket URL: <https://trac.macports.org/ticket/56107>
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