Fix build with Boost 1.89.0

[cho-m]

This fixes a regression in 0404f7c / 1.15.1 as Boost::system is not valid since Boost 1.89.0.

PCL requires a minimum of Boost 1.71.0 so Boost.System is guaranteed to be header-only (since 1.69¹) and the component can be removed.

---

Build failure was originally seen at Homebrew (Homebrew/homebrew-core#235073) when trying to update to 1.15.1

  -- Configuring done (32.4s)
  CMake Error at outofcore/CMakeLists.txt:79 (target_link_libraries):
    Target "pcl_outofcore" links to:
  
      Boost::system
  
    but the target was not found.  Possible reasons include:
  
      * There is a typo in the target name.
      * A find_package call is missing for an IMPORTED target.
      * An ALIAS target is missing.

Footnotes

1. https://www.boost.org/doc/libs/1_69_0/libs/system/doc/html/system.html#changes_in_boost_1_69 ↩

[larshg]

A shame it wasn't tested with the pre-release so it could have been fixed before 1.15.1 was released 😢

But thanks for the PR 😄

[mvieth]

A shame it wasn't tested with the pre-release so it could have been fixed before 1.15.1 was released 😢

I actually did test the 1.15.1 release candidate with Boost 1.89.0, but it was our last-minute change in #6325 that broke compatibility with Boost 1.89.0 (more specifically, changing ${Boost_SYSTEM_LIBRARY} to Boost::system).

[mvieth]

Thank you for the PR @cho-m

[larshg]

A shame it wasn't tested with the pre-release so it could have been fixed before 1.15.1 was released 😢

I actually did test the 1.15.1 release candidate with Boost 1.89.0, but it was our last-minute change in #6325 that broke compatibility with Boost 1.89.0 (more specifically, changing ${Boost_SYSTEM_LIBRARY} to Boost::system).

Ahh, yeah okay. That was unfortunate - somehow I remembered it as it was part of the release candidate 😆