[Bug bootstrap/64023] [5 Regression] r216964 breaks bootstrap on darwin when using gcc as the bootstrap compiler.

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64023

--- Comment #2 from Iain Sandoe <iains at gcc dot gnu.org> ---
as commented in 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63773 c#14..17
this is caused by
$(HOST_EXPORTS) being used at stage#N>1 instead of $(POSTSTAGE1_HOST_EXPORTS)

Thus the config test for the bootstrap compiler is affecting the later build. 
In this case, GCC fails to build because it supports -static-libstdc++ (and
thus the option is passed to the build) but it then needs the -B options to
allow for spec substitution to work for libstdc++.  These are not present in
HOST_EXPORTS because, obviously, the host compiler is installed and doesn't
need them.

NOTE: (clang succeeds because it does not support -staticlibstdc++, so that
option is not passed to the stage#3 libs build.
However in that case the fault is potentially more subtle, because the library
is then being linked with a dependency on the *system* libstdc++.dylib and not
the current build.