* docs/html/test.html: Mention PCH_CXXFLAGS.
	* testsuite/lib/libstdc++.exp: Set PCH_CXXFLAGS by probing for an
	available stcd++.h PCH.
	* testsuite/libstdc++-dg/normal.exp: Use PCH_CXXFLAGS.

From-SVN: r100264