Build warnings

[gunnarhj]

Hi!

In connection with a QA review in Ubuntu a couple of opencc build warnings were pointed out:

/<<BUILDDIR>>/opencc-1.1.1+git20200624+ds2/src/PhraseExtract.cpp:104:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<std::pair<opencc::UTF8StringSliceBase<unsigned char>, opencc::PhraseExtract::Signals> >::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
  104 | for (int i = 0; i < items.size(); i++) {
      | ~~^~~~~~~~~~~~~~

/usr/bin/ld: CMakeFiles/opencc.dir/CommandLine.cpp.o: in function `Convert(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
./obj-x86_64-linux-gnu/src/tools/./src/tools/CommandLine.cpp:89: warning: the use of `tmpnam' is dangerous, better use `mkstemp'

It would be great if somebody could address those to improve the robustness of the opencc project.

[BYVoid]

cf4b5df