Fix up make pippack command for building source package for PyPI (#3199)

* Now `make pippack` works without any manual action: it will produce
  xgboost-[version].tar.gz, which one can use by typing
  `pip3 install xgboost-[version].tar.gz`.
* Detect OpenMP-capable compilers (clang, gcc-5, gcc-7) on MacOS

python-package/xgboost/build-python.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # This is a simple script to make xgboost in MAC and Linux for python wrapper only
 # Basically, it first try to make with OpenMP, if fails, disable OpenMP and make it again.
 # This will automatically make xgboost for MAC users who don't have OpenMP support.
@@ -9,22 +9,44 @@
 # note: this script is build for python package only, and it might have some filename
 #       conflict with build.sh which is for everything. 
 
+set -e
+set -x
 
 #pushd xgboost
 oldpath=`pwd`
 cd ./xgboost/
+
+if echo "${OSTYPE}" | grep -q "darwin"; then
+  LIB_XGBOOST=libxgboost.dylib
+  # Use OpenMP-capable compiler if possible
+  if which g++-5; then
+    export CC=gcc-5
+    export CXX=g++-5
+  elif which g++-7; then
+    export CC=gcc-7
+    export CXX=g++-7
+  elif which clang++; then
+    export CC=clang
+    export CXX=clang++
+  fi
+else
+  LIB_XGBOOST=libxgboost.so
+fi
+
 #remove the pre-compiled .so and trigger the system's on-the-fly compiling
 make clean
-if make lib/libxgboost.so -j4; then
+if make lib/${LIB_XGBOOST} -j4; then
     echo "Successfully build multi-thread xgboost"
 else
     echo "-----------------------------"
     echo "Building multi-thread xgboost failed"
     echo "Start to build single-thread xgboost"
     make clean
-    make lib/libxgboost.so -j4 USE_OPENMP=0
+    make lib/${LIB_XGBOOST} -j4 USE_OPENMP=0
     echo "Successfully build single-thread xgboost"
     echo "If you want multi-threaded version"
     echo "See additional instructions in doc/build.md"
 fi
 cd $oldpath
+
+set +x