Make split sections by default work again

Authored by rwbarton on Feb 4 2017, 4:29 AM.

Description

Make split sections by default work again

Commit 266a9dc4c changed = to := in one place in mk/config.mk.in.
This broke SupportsSplitSections because the variable LdIsGNULd
that it depends on is not defined until later in the file.

Test Plan: pushed to a wip/ branch for perf to build

Reviewers: DemiMarie, austin, bgamari, mpickering

Reviewed By: mpickering

Subscribers: mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D3070

GHC Trac Issues: Trac #13230