gdb: enable -Wmissing-prototypes warning

While compiling with clang, I noticed it didn't catch cases where my
function declaration didn't match my function definition.  This is
normally caught by gcc with -Wmissing-declarations.

On clang, this is caught by -Wmissing-prototypes instead.

Note that on gcc, -Wmissing-prototypes also exists, but is only valid
for C and Objective-C.  It gets correctly rejected by the configure
script since gcc rejects it with:

    cc1plus: error: command line option '-Wmissing-prototypes' is valid for C/ObjC but not for C++ -Werror

So this warning flag ends up not used for gcc (which is what we want).

gdb/ChangeLog:

	* configure: Re-generate.

gdbserver/ChangeLog:

	* configure: Re-generate.

gdbsupport/ChangeLog:

	* configure: Re-generate.
	* warning.m4: Enable -Wmissing-prototypes.