compat: introduce new "zlib.h" header

Introduce a new "compat/zlib-compat.h" header that we include instead of
including <zlib.h> directly. This will allow us to wire up zlib-ng as an
alternative backend for zlib compression in a subsequent commit.

Note that we cannot just call the file "compat/zlib.h", as that may
otherwise cause us to include that file instead of <zlib.h>.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/zlib-compat.h

@@ -0,0 +1,6 @@
+#ifndef COMPAT_ZLIB_H
+#define COMPAT_ZLIB_H
+
+#include <zlib.h>
+
+#endif /* COMPAT_ZLIB_H */

git-compat-util.h

@@ -1537,7 +1537,7 @@ int cmd_main(int, const char **);
 int common_exit(const char *file, int line, int code);
 #define exit(code) exit(common_exit(__FILE__, __LINE__, (code)))
 
-#include <zlib.h>
+#include "compat/zlib-compat.h"
 
 /*
  * This include must come after system headers, since it introduces macros that

reftable/block.c

@@ -13,7 +13,7 @@ https://developers.google.com/open-source/licenses/bsd
 #include "record.h"
 #include "reftable-error.h"
 #include "system.h"
-#include <zlib.h>
+#include "compat/zlib-compat.h"
 
 int header_size(int version)
 {