PR c/21668
        * c-decl.c (grokdeclarator): Don't warn for 'extern const' when
        compiling at the intersection of C and C++.

testsuite/
        PR c/21668
        * gcc.dg/Wc++-compat.c: New.
        * gcc.dg/Wno-c++-compat.c: New.

From-SVN: r107401