[cmake] Fix for system-installed boost.

scripts/cmake/Dependencies.cmake

@@ -190,20 +190,21 @@ if(OGS_USE_MFRONT)
     endif()
 endif()
 
+# Boost libraries used by ogs, can be linked with Boost::[lib_name]
+set(BOOST_INCLUDE_LIBRARIES
+    math
+    property_tree
+    algorithm
+    smart_ptr
+    tokenizer
+    assign
+    dynamic_bitset
+    range
+    variant
+)
 if(GUIX_BUILD)
     find_package(Boost REQUIRED)
 else()
-    # Boost libraries used by ogs, can be linked with Boost::[lib_name]
-    set(BOOST_INCLUDE_LIBRARIES
-        math
-        property_tree
-        algorithm
-        smart_ptr
-        tokenizer
-        assign
-        dynamic_bitset
-        range
-    )
     CPMFindPackage(
         NAME Boost
         VERSION ${ogs.minimum_version.boost}
@@ -211,6 +212,13 @@ else()
         OPTIONS "BOOST_ENABLE_CMAKE ON"
     )
 endif()
+if(NOT Boost_ADDED)
+    # Boost from system found. There are only Boost::headers and Boost::boost
+    # targets.
+    foreach(lib ${BOOST_INCLUDE_LIBRARIES})
+        add_library(Boost::${lib} ALIAS Boost::headers)
+    endforeach()
+endif()
 
 CPMFindPackage(
     NAME LibXml2