PR libstdc++/69386
	* include/c_global/ccomplex: Ensure C++ language linkage.
	* include/c_global/cmath: Likewise.
	* include/c_global/cstdlib: Likewise.
	* include/c_global/ctgmath: Likewise.
	* testsuite/17_intro/headers/c++2011/linkage.cc: New.

From-SVN: r232607