[commit: ghc] master: configure: Catch case where LLVM tools can't be found (49c1a20)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/49c1a20dbbdabc8101877e8ef2c8de9ec3a079d0/ghc

>---------------------------------------------------------------

commit 49c1a20dbbdabc8101877e8ef2c8de9ec3a079d0
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Mon Sep 25 18:39:11 2017 -0400

    configure: Catch case where LLVM tools can't be found
    
    Previously we didn't specify the prog-not-found value passed to
    AC_CHECK_TOOLS.  Reported by @snowleopard in
    https://github.com/snowleopard/hadrian/issues/415.
    
    Test Plan: validate
    
    Reviewers: austin, hvr
    
    Subscribers: rwbarton, thomie, snowleopard, erikd
    
    Differential Revision: https://phabricator.haskell.org/D3992


>---------------------------------------------------------------

49c1a20dbbdabc8101877e8ef2c8de9ec3a079d0
 aclocal.m4 | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/aclocal.m4 b/aclocal.m4
index af84b6f..c89ef0d 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -2035,7 +2035,7 @@ AC_DEFUN([XCODE_VERSION],[
 #
 AC_DEFUN([FIND_LLVM_PROG],[
     # Test for program with and without version name.
-    AC_CHECK_TOOLS([$1], [$2-$3 $2])
+    AC_CHECK_TOOLS([$1], [$2-$3 $2], [:])
     if test "$$1" != ":"; then
         AC_MSG_CHECKING([$$1 is version $3])
         if test `$$1 --version | grep -c "version $3"` -gt 0 ; then
@@ -2044,6 +2044,8 @@ AC_DEFUN([FIND_LLVM_PROG],[
             AC_MSG_RESULT(no)
             $1=""
         fi
+    else
+        $1=""
     fi
 ])