re PR c/21342 (some incompatible external declarations not diagnosed)

re PR c/21342 (some incompatible external declarations not diagnosed)

	PR c/21342
	* c-decl.c (pushdecl): When there is a declaration in the current
	scope and the declarations are external linkage, check for
	compatibility with the type in the external scope and update the
	type in the external scope with the composite type information.
	Do not form a composite type of the new type and the visible type
	if they are incompatible.

testsuite:
	* gcc.dg/redecl-11.c, gcc.dg/redecl-12.c, gcc.dg/redecl-13.c,
	gcc.dg/redecl-14.c, gcc.dg/redecl-15.c: New tests.

From-SVN: r99510