Reenable FindLibXml2 on Windows and confirm its at least 2.8 or newer

[lanza]

Summary

GnuWin32 installs libxml2 2.4. This isn't recent enough for lldb's
usage and thus the build fails. Searching for libxml2 was disabled
due to this build failure.

However, the gdb-remote plugin requires libxml2 to parse various
gdb-remote protocol packets. Thus check and confirm that the libxml2
version is at least 2.8 and disable it if not.

[lanza]

@zturner With this the m_process->LoadModules() in DynamicLoaderWindowsDYLD.cpp:96 now queries the gdb-remote for the DLLs and thus can step into DLLs just fine.

lldb/trunk/cmake/modules/LLDBConfig.cmake

if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
   set_target_properties(lldb-headers PROPERTIES FOLDER "Misc")
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_llvm_install_targets(install-lldb-headers
                              COMPONENT lldb-headers)
   endif()
 endif()
 
-if (NOT LIBXML2_FOUND AND NOT (CMAKE_SYSTEM_NAME MATCHES "Windows"))
-  # Skip Libxml2 on Windows.  In CMake 3.4 and higher, the algorithm for
-  # finding libxml2 got "smarter", and it can now locate the version which is
-  # in gnuwin32, even though that version does not contain the headers that
-  # LLDB uses.
+if (NOT LIBXML2_FOUND)
   find_package(LibXml2)
 endif()
 
 # Find libraries or frameworks that may be needed
 if (APPLE)
   if(NOT IOS)
     find_library(CARBON_LIBRARY Carbon)
     find_library(CORE_SERVICES_LIBRARY CoreServices)
if (APPLE)
   list(APPEND system_libs xml2
        ${CURSES_LIBRARIES}
        ${FOUNDATION_LIBRARY}
        ${CORE_FOUNDATION_LIBRARY}
        ${CORE_SERVICES_LIBRARY}
        ${SECURITY_LIBRARY}
        ${DEBUG_SYMBOLS_LIBRARY})
   include_directories(${LIBXML2_INCLUDE_DIR})
-else()
-  if (LIBXML2_FOUND)
-    add_definitions( -DLIBXML2_DEFINED )
-    list(APPEND system_libs ${LIBXML2_LIBRARIES})
-    include_directories(${LIBXML2_INCLUDE_DIR})
-  endif()
-endif()
+elseif(LIBXML2_FOUND AND LIBXML2_VERSION_STRING VERSION_GREATER 2.8)
 
 if( WIN32 AND NOT CYGWIN )
   set(PURE_WINDOWS 1)
 endif()
 
 if(NOT PURE_WINDOWS)
   set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
   find_package(Threads REQUIRED)