Remove CMAKE_CXX_COMPILER and CMAKE_C_COMPILER for OSX (#719)

* Remove CMAKE_CXX_COMPILER and CMAKE_C_COMPILER for OSX

* update .travis/test.sh

* update again

.travis/test.sh

@@ -14,6 +14,15 @@ if [[ ${TASK} == "gpu" ]]; then
     export CPLUS_INCLUDE_PATH="$HOME/miniconda/include:$AMDAPPSDK/include/:$CPLUS_INCLUDE_PATH"
 fi
 
+case ${TRAVIS_OS_NAME} in
+    osx)
+        export CXX=g++-7
+        export CC=gcc-7
+        ;;
+    linux)
+        ;;
+esac
+
 cd $TRAVIS_BUILD_DIR
 
 if [[ ${TASK} == "pylint" ]]; then

CMakeLists.txt

 cmake_minimum_required(VERSION 2.8)
 
-if(APPLE)
-    SET(CMAKE_CXX_COMPILER "g++-7")
-    SET(CMAKE_C_COMPILER "gcc-7")
-endif()
-
 PROJECT(lightgbm)
 
 OPTION(USE_MPI "MPI based parallel learning" OFF)