Move Boost.Compute include path to the beginning of compiler search paths (#411)

* fix the size of vector cnt_per_class

* put Boost.Compute includes at the beginning

We want to put the submodule Boost.Compute at the beginning of compiler search
paths, because the submodule usually contains upstream bug fixes that have not
been included in the stable Boost releases installed on the host OS.

CMakeLists.txt

@@ -36,6 +36,8 @@ else()
 endif(USE_OPENMP)
 
 if(USE_GPU)
+    SET(BOOST_COMPUTE_HEADER_DIR ${PROJECT_SOURCE_DIR}/compute/include)
+    include_directories (${BOOST_COMPUTE_HEADER_DIR})
     find_package(OpenCL REQUIRED)
     include_directories(${OpenCL_INCLUDE_DIRS})
     MESSAGE(STATUS "OpenCL include directory:" ${OpenCL_INCLUDE_DIRS})
@@ -75,13 +77,11 @@ endif()
 
 
 SET(LightGBM_HEADER_DIR ${PROJECT_SOURCE_DIR}/include)
-SET(BOOST_COMPUTE_HEADER_DIR ${PROJECT_SOURCE_DIR}/compute/include)
 
 SET(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR})
 SET(LIBRARY_OUTPUT_PATH ${PROJECT_SOURCE_DIR})
 
 include_directories (${LightGBM_HEADER_DIR})
-include_directories (${BOOST_COMPUTE_HEADER_DIR})
 
 if(APPLE)
   if (APPLE_OUTPUT_DYLIB)