Fix the demo's compiling failure in some cases

Some demos based on CMake may search installed packages out of
Occlum toolchain's directory, this could cause some building
errors sometimes. By setting the $PKG_CONFIG_LIBDIR, we can make
CMake searching for packages in Occlum toolchain's directory only.

demos/grpc/download_and_install_grpc.sh

@@ -1,7 +1,9 @@
 #!/bin/sh
-INSTALL_DIR=/usr/local/occlum/x86_64-linux-musl/
+INSTALL_DIR=/usr/local/occlum/x86_64-linux-musl
 GRPC_SRC_DIR=$PWD/grpc
 export PATH=$PATH:$INSTALL_DIR/bin
+# Tell CMake to search for packages in Occlum toolchain's directory only
+export PKG_CONFIG_LIBDIR=$INSTALL_DIR/lib
 
 git clone https://github.com/grpc/grpc.git 
 cd grpc

demos/openvino/download_and_build_openvino.sh

@@ -36,7 +36,7 @@ build_opencv() {
       -DBUILD_opencv_python=OFF -DBUILD_PYTHON_SUPPORT=OFF \
       -DBUILD_EXAMPLES=OFF -DWITH_FFMPEG=OFF \
       -DWITH_QT=OFF -DWITH_CUDA=OFF
-    make -j
+    make -j4
     sudo make install
     popd
 }
@@ -105,6 +105,9 @@ while [ -n "$1" ]; do
     esac
 done
 
+# Tell CMake to search for packages in Occlum toolchain's directory only
+export PKG_CONFIG_LIBDIR=$PREFIX/lib
+
 if [ "$THREADING" == "TBB" ] ; then
     echo "Build OpenVINO with TBB threading"
     build_opencv

demos/openvino/install_cmake.sh

@@ -4,5 +4,5 @@ cd ~
 wget https://github.com/Kitware/CMake/releases/download/v3.15.5/cmake-3.15.5.tar.gz && tar xf cmake-3.15.5.tar.gz
 cd cmake-3.15.5
 ./bootstrap
-make -j
+make -j4
 sudo make install

demos/xgboost/README.md

@@ -2,7 +2,7 @@
 
 Step 1: Download XGBoost and its dependencies, and then build XGBoost
 ```
-./build_xgboost.sh
+./download_and_build_xgboost.sh
 ```
 When completed, the resulting XGBoost can be found in `xgboost_src` directory.
 

demos/xgboost/download_and_build_xgboost.sh

@@ -1,6 +1,9 @@
 #!/bin/bash
 set -e
 
+# Tell CMake to search for packages in Occlum toolchain's directory only
+export PKG_CONFIG_LIBDIR=/usr/local/occlum/x86_64-linux-musl/lib
+
 # Install the dependencies
 apt-get update
 apt-get install -y python3.5
@@ -30,7 +33,7 @@ cd build
 cmake ../ \
   -DCMAKE_BUILD_TYPE=Release \
   -DCMAKE_C_COMPILER=occlum-gcc -DCMAKE_CXX_COMPILER=occlum-g++
-make -j
+make -j4
 popd
 
 # Prepare data