external/x64: "make build_env=external" fails

[nyh]

We've upgraded the boost libraries to a version from Fedora 21, compiled with gcc 4.9. However, our external/../libstdc++.a is from gcc 4.8.2. These two cannot work together, currently make build_env=external fails with:

  LINK loader.elf
build/release.x64/core/mempool.o: In function `memory::mark_smp_allocator_intialized::mark_smp_allocator_intialized()':
/home/nyh/osv.tmp1/core/mempool.cc:365: undefined reference to `__cxa_throw_bad_array_new_length'
external/x64/misc.bin/usr/lib64//libboost_program_options.a(cmdline.o): In function `boost::program_options::detail::cmdline::parse_long_option(std::vector<std::string, std::allocator<std::string> >&)':
(.text+0x1c8a): undefined reference to `std::__throw_out_of_range_fmt(char const*, ...)'
...

These functions, like __throw_out_of_range_fmt, are new to gcc 4.9 and it expects them to exist in the C++ standard library, but because we have in external/ a libstdc++.a from gcc 4.8, it is missing this function.

So we need to upgrade external/libstdc++.a to one from gcc 4.9 too.

[nyh]

When the host compiler is gcc 5.1, "make build_env=external" now fails on the bug described in issue #632: the old Boost header files in external/ cannot compile on the newer compiler because of a problem in boost::addressof().

The obvious solution is not to use "make lib_env=external" at all - and even remove this option from our Makefile... But there is one seriosu flaw in this solution: Cross-compiling with "make arch=aarch64" requires this external... On Fedora (for example), we have a cross-compiler for aarch64, but the various libraries (Boost libraries, and even libstdc++) are not available. So we use lib_env=external when cross compiling.

[wkozaczuk]

@nyh,

Similarly as with #687, shall we close this one?

[wkozaczuk]

@nyh Closing for the reasons explained in #687 . I hope you agree this can be closed.