Fix finding the unordered_map header.

Apply the fix made in kdevplatform commit
d756d28f4b000d2b7af12822c935dd13754876b4.

Including unordered_map generates a warning when the compiler is not
passed the -std=c++0x option, which made the check fail when the
compilation CMake tried in check_include_file_cxx did not use it.