configure: Add proper check to see if object merging works

m4/find_merge_objects.m4

@@ -3,25 +3,59 @@
 # Find which linker to use to merge object files.
 #
 # See Note [Merging object files for GHCi] in GHC.Driver.Pipeline.
+
+
+AC_DEFUN([CHECK_MERGE_OBJECTS],[
+  AC_REQUIRE([FP_FIND_NM])
+  AC_MSG_NOTICE([Checking whether $MergeObjsCmd can merge objects])
+  echo 'int funA(int x) {return x;}' > conftesta.c
+  echo 'int funB(int x) {return x;}' > conftestb.c
+  "$CC" -c -o conftesta.o conftesta.c
+  "$CC" -c -o conftestb.o conftestb.c
+  $MergeObjsCmd $MergeObjsArgs conftesta.o conftestb.o -o conftestc.o || AC_MSG_ERROR([ $MergeObjsCmd could not merge objects ])
+
+  # Check the resulting object file has both functions.
+  "$NM" conftestc.o | grep funA > /dev/null 2>&1 || AC_MSG_ERROR([ $MergeObjsCmd could not merge objects ])
+  "$NM" conftestc.o | grep funB > /dev/null 2>&1 || AC_MSG_ERROR([ $MergeObjsCmd could not merge objects ])
+
+  rm -r conftest*.c conftest*.o
+])
+
 AC_DEFUN([FIND_MERGE_OBJECTS],[
     AC_REQUIRE([FIND_LD])
 
     if test -z ${MergeObjsCmd+x}; then
+        AC_MSG_NOTICE([Setting cmd])
         MergeObjsCmd="$(command -v $LD)"
     fi
     if test -z ${MergeObjsArgs+x}; then
         MergeObjsArgs="-r"
     fi
 
-    CHECK_FOR_GOLD_T22266($MergeObjsCmd)
-    if test "$result" = "1"; then
-        AC_MSG_NOTICE([$MergeObjsCmd is broken due to binutils 22266, looking for another linker...])
-        MergeObjsCmd=""
-        AC_CHECK_TARGET_TOOL([MergeObjsCmd], [ld])
-        CHECK_FOR_GOLD_T22266($MergeObjsCmd)
-        if test "$result" = "1"; then
-            AC_MSG_ERROR([Linker is affected by binutils 22266 but couldn't find another unaffected linker. Please set the MergeObjsCmd variable to a functional linker.])
-        fi
+
+    # If MergeObjsCmd="" then we assume that the user is explicitly telling us that
+    # they do not want to configure the MergeObjsCmd, this is particularly important for
+    # the bundled windows toolchain.
+    if test -z "$MergeObjsCmd"; then
+      AC_MSG_NOTICE([No command for merging objects as explicitly instructed by user])
+
+    else
+      # Check first that gold works
+      CHECK_FOR_GOLD_T22266($MergeObjsCmd)
+      if test "$result" = "1"; then
+          AC_MSG_NOTICE([$MergeObjsCmd is broken due to binutils 22266, looking for another linker...])
+          MergeObjsCmd=""
+          AC_CHECK_TARGET_TOOL([MergeObjsCmd], [ld])
+          CHECK_FOR_GOLD_T22266($MergeObjsCmd)
+          if test "$result" = "1"; then
+              AC_MSG_ERROR([Linker is affected by binutils 22266 but couldn't find another unaffected linker. Please set the MergeObjsCmd variable to a functional linker.])
+          fi
+
+      fi
+
+      # Now just check that merging objects works at all
+      CHECK_MERGE_OBJECTS()
+
     fi
 
     AC_SUBST([MergeObjsCmd])