#1331 Clangd-client: Fix template error and link with GTK2 on Linux

[Miguel Gimenez]

Now the C++17 requirement has been removed, compiling with Linux Mint gives a template error. This is because the last parameter should be a std::string but it is a wxString.

Regarding the change from GTK3 to GTK2, all projects for Linux use GTK2; This is the only one using GTK3. This requires installation of both toolchains,

[pecan]

Applied Head rev 13061
Thanks