Modernize build

• Uses XDT_VERSION_INIT(), obviating the need for a configure.ac.in -> configure.ac build step.
• Removes XDT_I18N() and intltool in favor of straight-up gettext.
• Adds a build target for compile_flags.txt in the root to help with clangd in vim error-checking and auto-complete setups.

See xfce4-dev-tools#41

cc: @skunnyk @Tamaranch