Fix assimp zlib finder.

Closes #49

CMake/FindAssimp.cmake

@@ -27,17 +27,19 @@ find_library(ASSIMP_LIBRARY
     assimp${LIBRARY_SUFFIX}d
     )
 
-find_package(ZLIB)
+set(ASSIMP_ZLIB_INCLUDE_DIR)
+find_package(ZLIB QUIET)
 if(NOT ZLIB_FOUND)
   find_library(ZLIB_LIBRARY
     NAMES
       zlibstaticd
       zlibstatic
     )
+  set(ASSIMP_ZLIB_INCLUDE_DIR "${ASSIMP_INCLUDE_DIR}")
+else()
+  set(ASSIMP_ZLIB_INCLUDE_DIR "${ZLIB_INCLUDE_DIR}")
 endif()
 
-set(ASSIMP_ZLIB_INCLUDE_DIR "${ASSIMP_INCLUDE_DIR}" "${ZLIB_INCLUDE_DIR}")
-
 set(ASSIMP_INCLUDE_DIRS "${ASSIMP_INCLUDE_DIR}")
 set(ASSIMP_LIBRARIES "${ASSIMP_LIBRARY}" "${ZLIB_LIBRARY}")