Fix flac_min/flac_max not defined with Emscripten.

When the compiler doesn't match the ones expected, flac_min/flac_max
aren't defined. There is a fallback to always define them, but MIN/MAX
were defined instead of flac_min/flac_max

This commit is the same as the one is the original FLAC project:
https://github.com/xiph/flac/blob/master/src/libFLAC/include/private/macros.h

third_party/FLAC/src/include/private/macros.h

@@ -61,12 +61,12 @@
 #define flac_min(a,b) __min(a,b)
 #endif
 
-#ifndef MIN
+#ifndef flac_min
-#define MIN(x,y)	((x) <= (y) ? (x) : (y))
+#define flac_min(x,y)	((x) <= (y) ? (x) : (y))
 #endif
 
-#ifndef MAX
+#ifndef flac_max
-#define MAX(x,y)	((x) >= (y) ? (x) : (y))
+#define flac_max(x,y)	((x) >= (y) ? (x) : (y))
 #endif
 
 #endif