Remove ability to override $LD.

Since autoconf always uses $CC to link C programs, allowing users to
override LD caused mismatches between what LD_LINK_IFELSE thought worked
and what ld thought worked.  If you do need to do this kind of thing you
need to set a compiler flag such as gcc's -fuse-ld in LDFLAGS.
diff --git a/INSTALL b/INSTALL
index 7f552bf..d527511 100644
--- a/INSTALL
+++ b/INSTALL
@@ -220,7 +220,7 @@
 can specify these as environment variables before running ./configure.
 For example:
 
-CFLAGS="-O -m486" LDFLAGS="-s" LIBS="-lrubbish" LD="/usr/foo/ld" ./configure
+CC="/usr/foo/cc" CFLAGS="-O" LDFLAGS="-s" LIBS="-lrubbish" ./configure
 
 3. Configuration
 ----------------