re PR c/39564 (variably modified function return types not always diagnosed)

	PR c/39564
	* c-decl.c (grokdeclarator): Diagnose declarations of functions
	with variably modified return type and no storage class
	specifiers, except for the case of nested functions.  Distinguish
	extern declarations of functions with variably modified return
	types from those of objects with variably modified types.

testsuite:
	* gcc.dg/vla-19.c: New test.

From-SVN: r146778