Thunks: Revisit implementation strategy for implicit library dependencies

#1880 added a libX11 dependency to libGL by creating a fake libX11 library that is discarded during installation. This carries some boilerplate code and applying it to other libraries in the future turns out somewhat laborious.

Instead, using the `--as-needed` flag mentioned in the original PR should work rather easily after all: We can just append `--as-needed libX11.so --no-as-needed` to the `LINK_FLAGS` property of libGL. If possible, `--push-state` should be used if all linkers we support for compiling thunks implement that flag, though.