cmake: scope Boost Test check to vcpkg

This check was added for vcpkg, given how it packages Boost. However, we
don't need to run the check for other platforms, and it's quite slow.
So, scope it to VCPKG. On my machine, this reduces the time to run
`cmake -B build` from ~12 seconds, to ~6 seconds.

Fixes: #30787

cmake/module/AddBoostIfNeeded.cmake

@@ -64,9 +64,9 @@ function(add_boost_if_needed)
     set(CMAKE_REQUIRED_DEFINITIONS)
   endif()
 
-  if(BUILD_TESTS)
+  # Some package managers, such as vcpkg, vendor Boost.Test separately
-    # Some package managers, such as vcpkg, vendor Boost.Test separately
+  # from the rest of the headers, so we have to check for it individually.
-    # from the rest of the headers, so we have to check for it individually.
+  if(BUILD_TESTS AND DEFINED VCPKG_TARGET_TRIPLET)
     list(APPEND CMAKE_REQUIRED_DEFINITIONS -DBOOST_TEST_NO_MAIN)
     include(CheckIncludeFileCXX)
     check_include_file_cxx(boost/test/included/unit_test.hpp HAVE_BOOST_INCLUDED_UNIT_TEST_H)