acinclude.m4 (GLIBCXX_ENABLE_PCH): Rework test such that it tests not only generation of pch files, but also their use.

* acinclude.m4 (GLIBCXX_ENABLE_PCH): Rework test such that it
tests not only generation of pch files, but also their use.
* aclocal.m4, configure: Rebuilt.

From-SVN: r69727