Pass autoconf triplets to sub-project configures

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie, erikd

Differential Revision: https://phabricator.haskell.org/D2713

GHC Trac Issues: #12840

configure.ac

@@ -437,7 +437,11 @@ then
 else
   CrossCompilePrefix=""
 fi
-TargetPlatformFull="${TargetPlatform}"
+# Despite its similarity in name to TargetPlatform, TargetPlatformFull is used
+# in calls to subproject configure scripts and thus must be set to the autoconf
+# triple, not the normalized GHC triple that TargetPlatform is set to.
+# It may be better to just do away with the GHC triples all together.
+TargetPlatformFull="${target}"
 AC_SUBST(CrossCompiling)
 AC_SUBST(CrossCompilePrefix)
 AC_SUBST(TargetPlatformFull)