build: if cross-compiling for an apple host, locate some additional tools

This should be safe to do for native builds too, but for now it's
specific to cross-builds to avoid possible regressions.

configure.ac

@@ -222,6 +222,7 @@ case $host in
      TARGET_OS=darwin
      LEVELDB_TARGET_FLAGS="TARGET_OS=Darwin"
      if  test x$cross_compiling != xyes; then
+       BUILD_OS=darwin
        AC_CHECK_PROG([PORT],port, port)
        if test x$PORT = xport; then
          dnl add default macports paths
@@ -238,6 +239,17 @@ case $host in
          CPPFLAGS="$CPPFLAGS -I$bdb_prefix/include"
          LIBS="$LIBS -L$bdb_prefix/lib"
        fi
+     else
+       case $build_os in
+         *darwin*)
+           BUILD_OS=darwin
+           ;;
+         *)
+           AC_PATH_TOOL([INSTALLNAMETOOL], [install_name_tool], install_name_tool)
+           AC_PATH_TOOL([OTOOL], [otool], otool)
+           AC_PATH_PROGS([GENISOIMAGE], [genisoimage mkisofs],genisoimage)
+           ;;
+       esac
      fi
 
      CPPFLAGS="$CPPFLAGS -DMAC_OSX"
@@ -671,6 +683,7 @@ if test "x$use_tests$build_bitcoind$use_qt" = "xnonono"; then
 fi
 
 AM_CONDITIONAL([TARGET_DARWIN], [test x$TARGET_OS = xdarwin])
+AM_CONDITIONAL([BUILD_DARWIN], [test x$BUILD_OS = xdarwin])
 AM_CONDITIONAL([TARGET_WINDOWS], [test x$TARGET_OS = xwindows])
 AM_CONDITIONAL([ENABLE_WALLET],[test x$enable_wallet == xyes])
 AM_CONDITIONAL([USE_QRCODE], [test x$use_qr = xyes])