plugins.texi: Document plugin_is_GPL_compatible.

plugins.texi: Document plugin_is_GPL_compatible.

	* doc/plugins.texi: Document plugin_is_GPL_compatible.
	* plugin.c (str_license): Declare.
	(try_init_one_plugin): Assert that the symbol
	'plugin_is_GPL_compatible' exists.

testsuite/ChangeLog

	* gcc.dg/plugin/selfassign.c: Declare plugin_is_GPL_compatible.
	* gcc.dg/plugin/ggcplug.c: Likewise.
	* gcc.dg/plugin/one_time_plugin.c: Likewise.
	* g++.dg/plugin/selfassign.c: Likewise.
	* g++.dg/plugin/attribute_plugin.c: Likewise.
	* g++.dg/plugin/dumb_plugin.c: Likewise.

From-SVN: r148667