Don't use lld when building with GCC

[slipher]

Pushing this change lying around before I forget how the rebasing stuff works.

I would also change the readme to say LLVM or GCC but will avoid conflicting with #20.

[illwieckz]

LGTM.

[illwieckz]

As a side note, this is already done in my CMake branch:

	# The -rpath-link argument is needed on Ubuntu/Precise to
	# avoid linker warnings about missing ld.linux.so.3.
	# TODO(sbc): remove this once we stop supporting Precise
	# as a build environment.
	# We (Dæmon) don't support Precise.
	# set_linker_flag("-fuse-ld=lld")

The CMake implementation is a by-hand translation, line by line, of the original SCons code. So I actually read everything from SCons that was re-implemented in CMake.