configure: Fix ar probed flags

FP_PROG_AR_ARGS appears to be probing for what flags to use to build an
archive with an index. However, none of the flags that it probed
previously were valid as they all started with `c`, which isn't a mode
character. This went unnoticed until someone attempted to use LLVM ar
since we have a special case for gnu ar.

Additionally, we work around a bug in llvm-ar 5.0 where ar will exit
with code 0 even if it fails to parse the command line. Silliness.

I believe these should rather all be mode `q` however I'll need to test
this on a few platforms to be certain.

Test Plan: Validate on OS X, BSD, and Linux with binutils and llvm ar

Reviewers: hvr

Subscribers: rwbarton, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D4214

aclocal.m4

@@ -1153,11 +1153,15 @@ if test $fp_prog_ar_is_gnu = yes; then
   fp_cv_prog_ar_args="q"
 else
   touch conftest.dummy
-  for fp_var in clqsZ clqs cqs clq cq ; do
+  for fp_var in qclsZ qcls qcs qcl qc ; do
      rm -f conftest.a
-     if "$fp_prog_ar" $fp_var conftest.a conftest.dummy > /dev/null 2> /dev/null; then
-        fp_cv_prog_ar_args=$fp_var
-        break
+     if "$fp_prog_ar" $fp_var conftest.a conftest.dummy > /dev/null 2> /dev/null ; then
+       # Also check that a result was created; it seems some llvm-ar versions
+       # exit with code zero even if they fail to parse the command line.
+       if test -f conftest.a ; then
+         fp_cv_prog_ar_args=$fp_var
+         break
+       fi
      fi
   done
   rm -f conftest*