[MacPorts] #57214: cargo links with libssh2 but does not declare dependency

[MacPorts]

#57214: cargo links with libssh2 but does not declare dependency
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  Reporter:  ryandesign  |      Owner:  (none)
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:
      Port:  cargo       |
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Comment (by l2dy):

 https://doc.rust-lang.org/cargo/reference/build-scripts.html

 > As an example to follow, let’s take a look at one of Cargo’s own
 dependencies, libgit2. The C library has a number of constraints:
 >
 > * It has an optional dependency on OpenSSL on Unix to implement the
 https transport.
 > * It has an optional dependency on ''libssh2'' on all platforms to
 implement the ssh transport.
 > * It is often not installed on all systems by default.
 > * It can be built from source using cmake.

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Ticket URL: <https://trac.macports.org/ticket/57214#comment:1>
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