configure: Add flag which turns ghc-toolchain check into error

We want to catch these errors in CI, but first we need to a flag which
turns this check into an error.

configure.ac

@@ -171,6 +171,14 @@ AC_ARG_ENABLE(ghc-toolchain,
 )
 AC_SUBST([EnableGhcToolchain])
 
+AC_ARG_ENABLE(strict-ghc-toolchain-check,
+[AS_HELP_STRING([--enable-strict-ghc-toolchain-check],
+                [Whether to raise an error if the output of ghc-toolchain differs from configure])],
+  [FP_CAPITALIZE_YES_NO(["$enableval"], [EnableStrictGhcToolchainCheck])],
+  [EnableStrictGhcToolchainCheck=NO]
+)
+AC_SUBST([EnableStrictGhcToolchainCheck])
+
 dnl CC_STAGE0, LD_STAGE0, AR_STAGE0 are like the "previous" variable
 dnl CC, LD, AR (inherited by CC_STAGE[123], etc.)
 dnl but instead used by stage0 for bootstrapping stage1

m4/ghc_toolchain.m4

@@ -151,6 +151,11 @@ $diff_output
 Don't worry! This won't affect your ghc in any way.
 However, in a near future, we will move to configuring toolchains with "ghc-toolchain" by default, so you might have discovered a future bug.
 In light of it, if you've spotted this difference, please report a GHC bug at https://www.haskell.org/ghc/reportabug])
+
+    case "$EnableStrictGhcToolchainCheck" in
+        YES)
+          AC_MSG_ERROR([Failing due to --enable-strict-ghc-toolchain-check])
+    esac
     fi
 ])