Use ${ZLIB_LIBRARY} instead of 'z' when linking against libz.

From Patrick Spendrin.

--HG--
branch : HEAD

lib/CMakeLists.txt

diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 21695b8..29ba802 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -142,7 +142,7 @@
 
 ADD_LIBRARY(zip SHARED ${LIBZIP_SOURCES} ${LIBZIP_EXTRA_FILES})
 SET_TARGET_PROPERTIES(zip PROPERTIES VERSION 2.0 SOVERSION 2 )
-TARGET_LINK_LIBRARIES(zip z)
+TARGET_LINK_LIBRARIES(zip ${ZLIB_LIBRARY})
 INSTALL(TARGETS zip
 	ARCHIVE DESTINATION lib
 	LIBRARY DESTINATION lib)