change the install prefix for capi

test=develop

paddle/scripts/paddle_build.sh

@@ -629,10 +629,10 @@ EOF
 
 function gen_capi_package() {
     if [[ ${WITH_C_API} == "ON" ]]; then
-        install_prefix="${PADDLE_ROOT}/build/capi_output"
-        rm -rf $install_prefix
-        make DESTDIR="$install_prefix" install
-        cd $install_prefix/usr/local
+        capi_install_prefix=${INSTALL_PREFIX:-/paddle/build}/capi_output
+        rm -rf $capi_install_prefix
+        make DESTDIR="$capi_install_prefix" install
+        cd $capi_install_prefix/
         ls | egrep -v "^Found.*item$" | xargs tar -czf ${PADDLE_ROOT}/build/paddle.tgz
     fi
 }