[PATCHv2] PR libstdc++/84654 Do not use __float128 if it is disabled by the compiler

[Jonathan Wakely]

On 12/03/18 16:02 -0300, Tulio Magno Quites Machado Filho wrote:
>Changes since v1:
> - Completely rewrite of the patch to set ENABLE_FLOAT128 at libstdc++
>   build time and undef _GLIBCXX_USE_FLOAT128 when building user code.
>
>--- 8< ---
>
>In order to use __float128 in C++ it's necessary to check if the
>compiler enabled its support too when building user code.
>This patch changes the behavior at libstdc++ build by setting
>ENABLE_FLOAT128, which is used to set the value of the exported macro
>_GLIBCXX_USE_FLOAT128.

Now that we're in stage 1 we can make this change. The patch looks
right so I'll go ahead and commit it to trunk (it shouldn't interfere
with testing any last-minute fixes needed for gcc-8).

Thanks for fixing this, Tulio.