build: keep -Wmissing-declarations and -Wmissing-prototypes for modern GCCs

Fixes a -Werror failure of xalloc.h used in src.
From Eric Blake.
http://lists.gnu.org/archive/html/bug-gnulib/2012-12/msg00006.html

* configure.ac: Check whether GCC pragma diagnostic push/pop works.
Enable these warnings for bison if it does.
Enable these warnings for the test suite anyway.