xdg.hpp: Fix -Wreturn-std-move
README: Add CI and Funding badges
.builds: Add CI testing on Arch Linux
xdg.hpp: Fix style
xdg.hpp: Throw when XDG_RUNTIME_DIR has bad perms

XDG_RUNTIME_DIR directory must be 0700, throws
xdg::BaseDirectoryException otherwise.
Makefile: Improve CXXFLAGS handling and add test
xdg_test: Add test for XDG_RUNTIME_DIR
xdg_test: Improve test utils functions
Initial commit