FindLibXml2: ensure consistent paths, version and flag information

As pkg-config is used as hint to perform the actual lookup of libraries
and include paths we may not find the pkg-config installation.
Mainly this makes sure the pkg-config information are used only if the
actual installation reported to the user is the one from pkg-config.

Fixes: #20149

Modules/FindLibXml2.cmake

@@ -52,7 +52,6 @@ The following cache variables may also be set:
 # in the find_path() and find_library() calls
 find_package(PkgConfig QUIET)
 PKG_CHECK_MODULES(PC_LIBXML QUIET libxml-2.0)
-set(LIBXML2_DEFINITIONS ${PC_LIBXML_CFLAGS_OTHER})
 
 find_path(LIBXML2_INCLUDE_DIR NAMES libxml/xpath.h
    HINTS
@@ -78,9 +77,7 @@ find_program(LIBXML2_XMLLINT_EXECUTABLE xmllint)
 # for backwards compat. with KDE 4.0.x:
 set(XMLLINT_EXECUTABLE "${LIBXML2_XMLLINT_EXECUTABLE}")
 
-if(PC_LIBXML_VERSION)
-    set(LIBXML2_VERSION_STRING ${PC_LIBXML_VERSION})
-elseif(LIBXML2_INCLUDE_DIR AND EXISTS "${LIBXML2_INCLUDE_DIR}/libxml/xmlversion.h")
+if(LIBXML2_INCLUDE_DIR AND EXISTS "${LIBXML2_INCLUDE_DIR}/libxml/xmlversion.h")
     file(STRINGS "${LIBXML2_INCLUDE_DIR}/libxml/xmlversion.h" libxml2_version_str
          REGEX "^#define[\t ]+LIBXML_DOTTED_VERSION[\t ]+\".*\"")
 
@@ -89,9 +86,20 @@ elseif(LIBXML2_INCLUDE_DIR AND EXISTS "${LIBXML2_INCLUDE_DIR}/libxml/xmlversion.
     unset(libxml2_version_str)
 endif()
 
-set(LIBXML2_INCLUDE_DIRS ${LIBXML2_INCLUDE_DIR} ${PC_LIBXML_INCLUDE_DIRS})
+set(LIBXML2_INCLUDE_DIRS ${LIBXML2_INCLUDE_DIR})
 set(LIBXML2_LIBRARIES ${LIBXML2_LIBRARY})
 
+# Did we find the same installation as pkg-config?
+# If so, use additional information from it.
+unset(LIBXML2_DEFINITIONS)
+foreach(libxml2_pc_lib_dir IN LISTS PC_LIBXML_LIBDIR PC_LIBXML_LIBRARY_DIRS)
+  if (LIBXML2_LIBRARY MATCHES "^${libxml2_pc_lib_dir}")
+    list(APPEND LIBXML2_INCLUDE_DIRS ${PC_LIBXML_INCLUDE_DIRS})
+    set(LIBXML2_DEFINITIONS ${PC_LIBXML_CFLAGS_OTHER})
+    break()
+  endif()
+endforeach()
+
 include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(LibXml2
                                   REQUIRED_VARS LIBXML2_LIBRARY LIBXML2_INCLUDE_DIR
@@ -102,6 +110,7 @@ mark_as_advanced(LIBXML2_INCLUDE_DIR LIBXML2_LIBRARY LIBXML2_XMLLINT_EXECUTABLE)
 if(LibXml2_FOUND AND NOT TARGET LibXml2::LibXml2)
   add_library(LibXml2::LibXml2 UNKNOWN IMPORTED)
   set_target_properties(LibXml2::LibXml2 PROPERTIES INTERFACE_INCLUDE_DIRECTORIES "${LIBXML2_INCLUDE_DIRS}")
+  set_target_properties(LibXml2::LibXml2 PROPERTIES INTERFACE_COMPILE_OPTIONS "${LIBXML2_DEFINITIONS}")
   set_property(TARGET LibXml2::LibXml2 APPEND PROPERTY IMPORTED_LOCATION "${LIBXML2_LIBRARY}")
 endif()