Automatically enable ThirdParty build of Zlib if not found.

Better searching for zlib in macports directory.
Fix to zlib source to work with OSX.
Added check that patch version is 7 or later
(we need 1.2.7, but only check the patch level at the moment)

cmake/ThirdPartyZlib.cmake

-OPTION(THIRDPARTY_BUILD_ZLIB "Build ZLib library" OFF)
+########################################################################
+#
+# ThirdParty configuration for Nektar++
+#
+# ZLib
+#
+########################################################################
 
+# Attempt to identify Macports libraries, if they exist. This prevents
+# cmake warnings later on.
+SET(ZLIB_ROOT /opt/local/lib)
+
+
+# Find a system ZLIB library
+# If not found enable the THIRDPARTY_BUILD_ZLIB option
+FIND_PACKAGE( ZLIB )
+IF (ZLIB_FOUND AND NOT ZLIB_VERSION_PATCH LESS 7 )
+    SET(ZLIB_LIBRARY ${ZLIB_LIBRARIES})
+    SET(ZLIB_LIBRARY_DEBUG ${ZLIB_LIBRARIES})
+    SET(ZLIB_LIBRARY_RELEASE ${ZLIB_LIBRARIES})
+    MESSAGE(STATUS "Found Zlib library: ${ZLIB_LIBRARY}")
+    OPTION(THIRDPARTY_BUILD_ZLIB "Build ZLib library" OFF)
+ELSE ()
+    OPTION(THIRDPARTY_BUILD_ZLIB "Build ZLib library" ON)
+ENDIF()
+
+
+# If we or the user
 IF (THIRDPARTY_BUILD_ZLIB)
     # Build the Zlib library separately
     EXTERNALPROJECT_ADD(
         zlib-1.2.7
         PREFIX ${TPSRC}
         URL ${TPURL}/zlib-1.2.7.tar.gz
-        URL_MD5 "60df6a37c56e7c1366cca812414f7b85"
+        URL_MD5 "4a162e0f643232e7e278d59a0603ceb0"
         DOWNLOAD_DIR ${TPSRC}
         CONFIGURE_COMMAND ${CMAKE_COMMAND} -DCMAKE_INSTALL_PREFIX:PATH=${TPSRC}/dist -DCMAKE_C_FLAGS:STRING=-fPIC ${TPSRC}/src/zlib-1.2.7
     )
     SET(ZLIB_LIBRARY z)
     SET(ZLIB_LIBRARY_DEBUG z)
     SET(ZLIB_LIBRARY_RELEASE z)
-ELSE (THIRDPARTY_BUILD_ZLIB)
-  # Attempt to identify Macports libraries, if they exist. This prevents
-  # cmake warnings later on.
-  IF (APPLE)
-    FIND_LIBRARY(ZLIB_LIBRARY z PATHS /opt/local/lib NO_DEFAULT_PATH)
-  ENDIF()
-  
-  # Find a system ZLIB library and use that instead
-  IF (ZLIB_LIBRARY-NOTFOUND OR NOT APPLE) 
-    FIND_PACKAGE( ZLIB )
-    IF (ZLIB_FOUND)
-      SET(ZLIB_LIBRARY ${ZLIB_LIBRARIES})
-    ENDIF()
-  ENDIF()
-  
-  #  IF (ZLIB_LIBRARY)
-  #  SET(Boost_ZLIB_LIBRARY ${ZLIB_LIBRARY})
-  #  SET(Boost_ZLIB_LIBRARY_RELEASE ${ZLIB_LIBRARY})
-  #  SET(Boost_ZLIB_LIBRARY_DEBUG ${ZLIB_LIBRARY})
-  #ENDIF()
-  
-  IF (ZLIB_FOUND)
-    MESSAGE(STATUS "Found Zlib library: ${ZLIB_LIBRARY}")
-  ENDIF ()
-  
 ENDIF (THIRDPARTY_BUILD_ZLIB)
 
-