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.

hyper.h

@@ -6,6 +6,8 @@
 #define VERNUM 10402
 #define VERNUM_HEX 0xA0B2
 
+#include <stdarg.h>
+
 namespace hr {
 
 using namespace std;
@@ -368,7 +370,7 @@ string its(int i);
 int hrand(int i);
 
 #ifndef STDSIZE
-template<class T> int size(const T& x) {return int(x.size()); }
+template<class T> int size(const T& x) {return x.size(); }
 #endif
 
 // initialize the achievement system.
@@ -3432,7 +3434,15 @@ int score_default(int id);
 void handle_event(SDL_Event& ev);
 
 #ifndef XPRINTF
-template<class...T> void Xprintf(const char *fmt, T... t) { printf(fmt, t...); }
+#ifdef __GNUC__
+__attribute__((__format__ (__printf__, 1, 2)))
+#endif
+void Xprintf(const char *fmt, ...) {
+    va_list ap;
+    va_start(ap, fmt);
+    vprintf(fmt, ap);
+    va_end(ap);
+}
 #endif
 
 void pop_game();
@@ -3502,4 +3512,4 @@ void gdpush(int t);
 
 extern int fontscale;
 
-}
+}