unlink-busy does not support cross-compilation

[Bruno Haible]

Hi Jim,

The module 'unlink-busy' does not support cross-compilation: It uses
AC_RUN_IFELSE, thus calling the compiler for the host, and then sets
INSTALL, which is meant to point to a command to execute on the build machine.

It should IMO better use the compiler for the build machine, but this
isn't easily available in Autoconf.

Or just replace this entire macro outright with

  AC_REQUIRE([AC_CANONICAL_BUILD])
  case "$build_os" in
    hpux* | mingw*) INSTALL=$ac_install_sh ;;
  esac

And why isn't the determination of the INSTALL variable part of Autoconf
proper?

What do you think?

Bruno