build: Add EXTRA_LDFLAGS environment variable handling (fixes #5999) (#6000)
build: Add EXTRA_LDFLAGS environment variable handling (fixes #5999) (#6000)

Allow extending LDFLAGS by setting EXTRA_LDFLAGS to be able to pass
-extldflags=-zrelro -ldflags=-extldflags=-znow for Arch Linux packaging
to get full relro.

wweich/syncthing,syncthing/syncthing,imsodin/syncthing,wweich/syncthing,syncthing/syncthing,wweich/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,imsodin/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,imsodin/syncthing,imsodin/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,imsodin/syncthing,wweich/syncthing