do not hardcode LD=ld in GNUmakefile

[Tom Gillespie]

When cross compiling for an alternate libc implementation (e.g. musl for
static linking) a different linker required. It is currenly impossible
to pass LD=some-other-linker-ld in the environment or as part of a call
to make because the GNUmakefile hardcodes LD=ld. This commit fixes that
an also make it so that make-target-contrib.sh inhertis LD in the same
way that it inherits CC.

I'm to 100% sure that this works as expected. I've tried it in what
should be a clean environment without issue, but am not entirely sure
that LD will be set correctly by default.

[Shawn Ellis (shawn-ellis17)]

I ran into something similar when trying to build SBCL on windows with Cygwin and CC=x86_64-w64-mingw32-gcc. The config file src/runtime/Config.x86-64-win32 uses CC=gcc even though I was trying to build with Mingw32. I was able to get around it by modifying the file to use CC ?= gcc.

I didn't try it, but using LD ?= ld might work for you in src/runtime/GNUmakefile.

[Douglas Katzman (dougk)]

I don't think LD should be set at all. The only uses occur in the win32 configurations and in the rule to make sbcl.mk, which also should not be setting LD.

[Douglas Katzman (dougk)]

this is obsolete. The makefile never directly uses 'ld' except in Config.*win32