Add a zlib support shell

Some versions of zlib don't have a deflateBound defined, so
we define it ourselves after including zlib.h.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

include/git/zlib.h

+/*
+ * This file is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License, version 2,
+ * as published by the Free Software Foundation.
+ *
+ * In addition to the permissions in the GNU General Public License,
+ * the authors give you unlimited permission to link the compiled
+ * version of this file into combinations with other programs,
+ * and to distribute those combinations without any restriction
+ * coming from the use of this file.  (The General Public License
+ * restrictions do apply in other respects; for example, they cover
+ * modification of the file, and distribution when not linked into
+ * a combined executable.)
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef INCLUDE_git_zlib_h__
+#define INCLUDE_git_zlib_h__
+
+#include "git/common.h"
+#include <zlib.h>
+
+/**
+ * @file git/zlib.h
+ * @brief Git data compression routines
+ * @defgroup git_zlib Git data compression routines
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+#if defined(NO_DEFLATE_BOUND) || ZLIB_VERNUM < 0x1200
+/**
+ * deflateBound returns an upper bound on the compressed size.
+ *
+ * This is a stub function used when zlib does not supply the
+ * deflateBound() implementation itself.
+ *
+ * @param stream the stream pointer.
+ * @param s total length of the source data (in bytes). 
+ * @return maximum length of the compressed data.
+ */
+GIT_INLINE(size_t) deflateBound(z_streamp stream, size_t s)
+{
+	return (s + ((s + 7) >> 3) + ((s + 63) >> 6) + 11);
+}
+#endif
+
+/** @} */
+GIT_END_DECL
+#endif