libphobos: Add target-zlib to top-level configure

libphobos: Add target-zlib to top-level configure

Removes the building of zlib from within libphobos, using instead the
libz_convenience.a library.

ChangeLog:

2019-04-12  Iain Buclaw  <ibuclaw@gdcproject.org>

	* configure.ac: Add target-zlib to target_libraries.
	* configure: Regenerate.

gcc/ChangeLog:

2019-04-12  Iain Buclaw  <ibuclaw@gdcproject.org>

	* doc/install.texi: Document --with-target-system-zlib.

libphobos/ChangeLog:

2019-04-12  Iain Buclaw  <ibuclaw@gdcproject.org>

	* m4/druntime/libraries.m4 (DRUNTIME_LIBRARIES_ZLIB): Use
	libz_convenience.a if not using system zlib.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* libdruntime/Makefile.in: Regenerate.
	* src/Makefile.am: Remove ZLIB_CSOURCES and AM_CFLAGS.
	* src/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.

From-SVN: r270328