[cpm] Add Boost as header-only.

scripts/cmake/Dependencies.cmake

@@ -130,7 +130,13 @@ CPMFindPackage(
     NAME Boost
     GITHUB_REPOSITORY Orphis/boost-cmake
     VERSION ${ogs.minimum_version.boost}
+    DOWNLOAD_ONLY YES
 )
+if(Boost_ADDED)
+    add_library(Boost::boost INTERFACE IMPORTED)
+    string(REPLACE "." "_" boost_version ${ogs.minimum_version.boost})
+    target_include_directories(Boost::boost SYSTEM INTERFACE ${Boost_SOURCE_DIR}/boost/boost_${boost_version})
+endif()
 
 if(OGS_USE_XDMF)
     # ZLIB is a HDF5 dependency