configure: check target (not build) understands -no_compact_unwind

Previously, we were branching on whether the build system was darwin to
shortcut this check, but we really want to branch on whether the target
system (which is what we are configuring ld_prog for) is darwin.

m4/fp_prog_ld_no_compact_unwind.m4

@@ -6,7 +6,7 @@ AC_DEFUN([FP_PROG_LD_NO_COMPACT_UNWIND],
 [
 AC_CACHE_CHECK([whether ld understands -no_compact_unwind], [fp_cv_ld_no_compact_unwind],
 [
-case $build in
+case $target in
   *-darwin)
     echo 'int foo() { return 0; }' > conftest.c
     "${CC-cc}" -c conftest.c