Set cmake minimum version to 3.0.2.

This version documents BUILD_SHARED_LIBS which we use.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 705c61e..b089c7d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,7 +2,7 @@
 # 1. Problems generating html pages.
 # 2. create usable libtool .la file
 
-CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
+CMAKE_MINIMUM_REQUIRED(VERSION 3.0.2)
 
 PROJECT(libzip C)