pkgs/build-support/cabal: the configure-phase warning check can be

disabled by setting 'strictConfigurePhase' to 'false'

This is necessary for some packages, like dns, because cabal warns about
multiple versions of the same dependency being used, but the usage is fine,
actually, so we want the build to succeed. Packages that depend on 'doctest'
also have this issue <https://github.com/sol/doctest-haskell/issues/69>.

pkgs/build-support/cabal/default.nix

@@ -141,6 +141,10 @@ assert !enableStaticLibraries -> versionOlder "7.7" ghc.version;
             # and run any regression test suites the package might have
             doCheck = enableCheckPhase;
 
+            # abort the build if the configure phase detects that the package
+            # depends on multiple versions of the same build input
+            strictConfigurePhase = true;
+
             # pass the '--enable-library-vanilla' flag to cabal in the
             # configure stage to enable building shared libraries
             inherit enableStaticLibraries;
@@ -198,11 +202,13 @@ assert !enableStaticLibraries -> versionOlder "7.7" ghc.version;
               echo "configure flags: $extraConfigureFlags $configureFlags"
               ./Setup configure --verbose --prefix="$out" --libdir='$prefix/lib/$compiler' \
                 --libsubdir='$pkgid' $extraConfigureFlags $configureFlags 2>&1 \
+              ${optionalString self.strictConfigurePhase ''
                 | ${coreutils}/bin/tee "$NIX_BUILD_TOP/cabal-configure.log"
                 if ${gnugrep}/bin/egrep -q '^Warning:.*depends on multiple versions' "$NIX_BUILD_TOP/cabal-configure.log"; then
                   echo >&2 "*** abort because of serious configure-time warning from Cabal"
                   exit 1
                 fi
+              ''}
 
               eval "$postConfigure"
             '';