Minor fixes for Mac and MinGW.

Eliminate a format-string warning by using less template magic:
https://stackoverflow.com/questions/12573968/how-to-use-gccs-printf-format-attribute-with-c11-variadic-templates

Fix one `%Ld` in `rogueviz.cpp` that should have been `%lld`.

Rename `savepng.c` into `savepng.cpp` so that we don't have to care about
the system's C compiler; it builds fine using the same C++ options as
HyperRogue itself.

Figure out how to get `-DCAP_PNG` and `-DCAP_ROGUEVIZ` working on Mac,
and document them.
Assume that roughly the same things should also work on MinGW.

Consistency on `STDSIZE`, even though it doesn't suffice for C++17 conformance.
It *almost* suffices; the one place where Clang still complains even with
`-std=c++17 -DSTDSIZE -Wno-sign-compare` is here:

    ./fieldpattern.cpp:757:51: error: non-constant-expression cannot be narrowed from type 'unsigned long' to 'int' in initializer list
          [-Wc++11-narrowing]
          ex.primes.emplace_back(primeinfo{nextprime, size(fp.matrices) / S7, (bool) fp.wsquare});
                                                      ^~~~~~~~~~~~~~~~~~~~~~

So, we fix that up too, in this patch.

rogueviz-video.cpp

@@ -23,7 +23,7 @@ void rvvideo(const string &fname) {
         reached = (2*reached-1) / 3;
       else reached *= 2;
       }
-    printf("reached = %Ld\n", reached);
+    printf("reached = %lld\n", reached);
     vector<string> seq;
     while(reached>1) { 
       seq.push_back(llts(reached));