libstdc++: Introduce _GLIBCXX_EXPLICIT_THIS_PARAMETER internal FTM

libstdc++: Introduce _GLIBCXX_EXPLICIT_THIS_PARAMETER internal FTM

This FTM is like __cpp_explicit_this_parameter but is also defined
in earlier C++ modes if deducing this is supported as an extension
by the compiler.  Currently only GCC supports this, Clang doesn't.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_EXPLICIT_THIS_PARAMETER):
	New.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>