cos: move cmake prefix path after external building

contrib/CMakeLists.txt

@@ -125,7 +125,6 @@ endif(${BUILD_WITH_SQLITE})
 # cos
 if(${BUILD_WITH_COS})
     file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/build/)
-    set(CMAKE_PREFIX_PATH ${CMAKE_BINARY_DIR}/build)
     cat("${TD_SUPPORT_DIR}/mxml_CMakeLists.txt.in" ${CONTRIB_TMP_FILE})
     cat("${TD_SUPPORT_DIR}/cos_CMakeLists.txt.in" ${CONTRIB_TMP_FILE})
     add_definitions(-DUSE_COS)
@@ -374,6 +373,7 @@ endif()
 # cos
 if(${BUILD_WITH_COS})
   if(NOT ${TD_WINDOWS})
+    set(CMAKE_PREFIX_PATH ${CMAKE_BINARY_DIR}/build)
     #ADD_DEFINITIONS(-DMINIXML_LIBRARY=${CMAKE_BINARY_DIR}/build/lib/libxml.a)
     option(ENABLE_TEST "Enable the tests" OFF)
     INCLUDE_DIRECTORIES(${CMAKE_BINARY_DIR}/build/include)