cmake: Fix build with libxslt and libxml2 as subprojects

Fixes https://gitlab.gnome.org/GNOME/libxml2/-/issues/416

CMakeLists.txt

@@ -32,4 +32,5 @@
 include(CMakePackageConfigHelpers)
 include(GNUInstallDirs)
 
+if(NOT TARGET LibXml2)
 	find_package(LibXml2 CONFIG REQUIRED)
@@ -35,4 +36,5 @@
 	find_package(LibXml2 CONFIG REQUIRED)
+endif()
 
 option(BUILD_SHARED_LIBS "Build shared libraries" ON)
 option(LIBXSLT_WITH_DEBUGGER "Add the debugging support" ON)