haiku: non-x86 spins use a normal libstdc++ filename. Handle the differences.

Fixes Bugzilla #3730.

configure.in

@@ -3534,7 +3534,10 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         fi
         # The Haiku platform requires special setup.
         SOURCES="$srcdir/src/main/haiku/*.cc $SOURCES"
-        EXTRA_LDFLAGS="$EXTRA_LDFLAGS -lroot -lbe -lmedia -lgame -ldevice -ltextencoding -lstdc++.r4"
+        EXTRA_LDFLAGS="$EXTRA_LDFLAGS -lroot -lbe -lmedia -lgame -ldevice -ltextencoding"
+        # Haiku's x86 spins use libstdc++.r4.so (for binary compat?), but
+        #  other spins, like x86-64, use a more standard "libstdc++.so.*"
+        AC_CHECK_FILE("/boot/system/lib/libstdc++.r4.so", EXTRA_LDFLAGS="$EXTRA_LDFLAGS -lstdc++.r4", EXTRA_LDFLAGS="$EXTRA_LDFLAGS -lstdc++")
         ;;
     arm*-apple-darwin*|*-ios-*)
         ARCH=ios