gnu: gcc: Fix libstdc++ misconfiguration.

Fixes <https://bugs.gnu.org/42392>.
Reported by Maxim Cournoyer and Erik Garrison.

Until now, the generated <bits/c++config.h> would be incorrect (many
_GLIBCXX_HAVE macros would be undefined), which in turn would lead to
build failures for most C++ programs.

* gnu/packages/gcc.scm (gcc-6)[inputs]: Add LIBSTDC++-HEADERS.
(libstdc++, libstdc++-headers): New variables.
* gnu/packages/gcc.scm (gfortran): Change first argument to 'custom-gcc'
to remove dependency on LIBSTDC++-HEADERS.