General improvements and fixes

[ArashPartow]

Added fixes for a clean GCC 7.+ compile

[zonkmachine]

Thanks for the update!

[ArashPartow]

You're welcome! :D

[tresf]

@ArashPartow we're hitting some compilation issues on Travis-CI with Mac/Clang with the version from your branch (we just switched).

Is our version different than yours? https://travis-ci.org/LMMS/lmms/jobs/300470102

Edit: Yes it is different. Our version has const removed. More info here: #3965

e.g.

/Users/travis/build/LMMS/lmms/plugins/Xpressive/exprtk/exprtk.hpp:1965:64: error: default initialization of an object of const type 'const typename numeric::details::number_type<double>::type' (aka 'const exprtk::details::numeric::details::real_type_tag') without a user-provided default constructor
         const typename numeric::details::number_type<T>::type num_type;
                                                               ^
                                                                       {}
/Users/travis/build/LMMS/lmms/plugins/Xpressive/exprtk/exprtk.hpp:3460:41: note: in instantiation of function template specialization 'exprtk::details::string_to_real<double>' requested here
                  if (!exprtk::details::string_to_real(t.value,v))
                                        ^
/Users/travis/build/LMMS/lmms/plugins/Xpressive/exprtk/exprtk.hpp:1965:64: error: default initialization of an object of const type 'const typename numeric::details::number_type<float>::type' (aka 'const exprtk::details::numeric::details::real_type_tag') without a user-provided default constructor
         const typename numeric::details::number_type<T>::type num_type;
                                                               ^