libphobos: Fix configure test for backtrace-supported.h

libphobos/ChangeLog:

2019-04-16  Iain Buclaw  <ibuclaw@gdcproject.org>

	* config.h.in: Regenerate.
	* configure: Regenerate.
	* m4/druntime/libraries.m4 (DRUNTIME_LIBRARIES_BACKTRACE): Set
	CPPFLAGS correctly for backtrace support test.

From-SVN: r270377