https://github.com/gentoo-haskell/gentoo-haskell/issues/959

From 144abba394c6a23eb877fbde727111a5e75b9d47 Mon Sep 17 00:00:00 2001
From: Ben Gamari <ben@smart-cactus.org>
Date: Wed, 18 Sep 2019 23:50:20 -0400
Subject: [PATCH] configure: Don't depend upon alex in source dist build

This fixes #16860 by verifying that the generated sources don't already
exist before asserting that the `alex` executable was found. This
replicates the logic already used for `happy` in the case of `alex`.
---
 aclocal.m4 | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/aclocal.m4
+++ b/aclocal.m4
@@ -984,8 +984,11 @@ else
 fi;
 changequote([, ])dnl
 ])
-FP_COMPARE_VERSIONS([$fptools_cv_alex_version],[-lt],[3.1.7],
-  [AC_MSG_ERROR([Alex version 3.1.7 or later is required to compile GHC.])])[]
+if test ! -f compiler/parser/Lexer.hs
+then
+    FP_COMPARE_VERSIONS([$fptools_cv_alex_version],[-lt],[3.1.7],
+      [AC_MSG_ERROR([Alex version 3.1.7 or later is required to compile GHC.])])[]
+fi
 AlexVersion=$fptools_cv_alex_version;
 AC_SUBST(AlexVersion)
 ])
-- 
2.23.0