Plugin support on Windows/MinGW

Plugin support on Windows/MinGW

config/ChangeLog:
2017-11-14 Boris Kolpackov  <boris@codesynthesis.com>

	* gcc-plugin.m4: Add support for MinGW.

gcc/ChangeLog:
2017-11-14 Boris Kolpackov  <boris@codesynthesis.com>

	* plugin.c (add_new_plugin): Use platform-specific library extensions.
	(try_init_one_plugin): Alternative implementation for MinGW.
	* Makefile.in (plugin_implib): New.
	(gengtype-lex.c): Fix broken AIX workaround.
	* configure: Regenerate.
	* doc/plugins.texi: Document support for MinGW.

gcc/c/ChangeLog:
2017-11-14 Boris Kolpackov  <boris@codesynthesis.com>

	* Make-lang.in (c.install-plugin): Install backend import library.

gcc/cp/ChangeLog:
2017-11-14 Boris Kolpackov  <boris@codesynthesis.com>

	* Make-lang.in (c++.install-plugin): Install backend import library.

libcc1/ChangeLog:
2017-11-14 Boris Kolpackov  <boris@codesynthesis.com>

	* configure: Regenerate.

From-SVN: r255154