Don't hard-code ld when extracting host linker version, use ${LD} if
it is set.

llvm-svn: 209742
diff --git a/llvm/configure b/llvm/configure
index e1959df..8c6270d 100755
--- a/llvm/configure
+++ b/llvm/configure
@@ -7612,7 +7612,7 @@
   echo $ECHO_N "(cached) $ECHO_C" >&6
 else
 
-   version_string="$(ld -v 2>&1 | head -1)"
+   version_string="$(${LD:-ld} -v 2>&1 | head -1)"
 
    # Check for ld64.
    if (echo "$version_string" | grep -q "ld64"); then