Add ZIP_STATIC for Windows static library compilation, and add a
paragraph in README explaining it.

README

diff --git a/README b/README
index 756a282..de9171f 100644
--- a/README
+++ b/README
@@ -17,6 +17,9 @@
 with most operating systems nowadays, or you can get it at
 	http://www.gzip.org/zlib/
 
+  When using a static Windows library, you need to define ZIP_STATIC
+when compiling packages using libzip.
+
   If you make a binary distribution, please include a pointer to the
 distribution site:
 	http://www.nih.at/libzip/

lib/zip.h

diff --git a/lib/zip.h b/lib/zip.h
index 050db76..ba38ac8 100644
--- a/lib/zip.h
+++ b/lib/zip.h
@@ -3,7 +3,7 @@
 
 /*
   zip.h -- exported declarations.
-  Copyright (C) 1999-2012 Dieter Baron and Thomas Klausner
+  Copyright (C) 1999-2014 Dieter Baron and Thomas Klausner
 
   This file is part of libzip, a library to manipulate ZIP archives.
   The authors can be contacted at <libzip@nih.at>
@@ -36,13 +36,17 @@
 
 
 #ifndef ZIP_EXTERN
-#ifdef _WIN32
-#define ZIP_EXTERN __declspec(dllimport)
-#elif defined(__GNUC__) && __GNUC__ >= 4
-#define ZIP_EXTERN __attribute__ ((visibility ("default")))
-#else
-#define ZIP_EXTERN
-#endif
+# ifndef ZIP_STATIC
+#  ifdef _WIN32
+#   define ZIP_EXTERN __declspec(dllimport)
+#  elif defined(__GNUC__) && __GNUC__ >= 4
+#   define ZIP_EXTERN __attribute__ ((visibility ("default")))
+#  else
+#   define ZIP_EXTERN
+#  endif
+# else
+#  define ZIP_EXTERN
+# endif
 #endif
 
 #ifdef __cplusplus