Merge branch 'ab/auto-detect-zlib-compress2'

The build procedure has been taught to notice older version of zlib
and enable our replacement uncompress2() automatically.

* ab/auto-detect-zlib-compress2:
  compat: auto-detect if zlib has uncompress2()

git-compat-util.h

@@ -1398,6 +1398,18 @@ void unleak_memory(const void *ptr, size_t len);
 #define UNLEAK(var) do {} while (0)
 #endif
 
+#define z_const
+#include <zlib.h>
+
+#if ZLIB_VERNUM < 0x1290
+/*
+ * This is uncompress2, which is only available in zlib >= 1.2.9
+ * (released as of early 2017). See compat/zlib-uncompress2.c.
+ */
+int uncompress2(Bytef *dest, uLongf *destLen, const Bytef *source,
+		uLong *sourceLen);
+#endif
+
 /*
  * This include must come after system headers, since it introduces macros that
  * replace system names.