Pass in CXX to libffi's configure script.

Reviewers: austin

Reviewed By: austin

Subscribers: thomie, carter, ezyang, simonmar

Differential Revision: https://phabricator.haskell.org/D370

GHC Trac Issues: #9720

libffi/ghc.mk

@@ -90,6 +90,7 @@ $(libffi_STAMP_CONFIGURE): $(TOUCH_DEP)
 	    $(LIBFFI_PATH_MANGLE) \
 	    cd build && \
 	    CC=$(CC_STAGE1) \
+	    CXX=$(CC_STAGE1) \
 	    LD=$(LD) \
 	    AR=$(AR_STAGE1) \
 	    NM=$(NM) \