Always build Arrow libraries when either C++/Python affected. Add missing ARROW_HOME env var to Parquet build CI script. [force travis] [skip appveyor]

Change-Id: Ib0153b039ebbab10af2db69695585c2280505ad4

.travis.yml

@@ -61,7 +61,8 @@ matrix:
     - $TRAVIS_BUILD_DIR/ci/travis_install_linux.sh
     - $TRAVIS_BUILD_DIR/ci/travis_install_clang_tools.sh
     - $TRAVIS_BUILD_DIR/ci/travis_lint.sh
-    - if [ $ARROW_CI_CPP_AFFECTED == "1" ]; then $TRAVIS_BUILD_DIR/ci/travis_before_script_cpp.sh; fi
+    # If either C++ or Python changed, we must install the C++ libraries
+    - $TRAVIS_BUILD_DIR/ci/travis_before_script_cpp.sh
     script:
     - if [ $ARROW_CI_CPP_AFFECTED == "1" ]; then $TRAVIS_BUILD_DIR/ci/travis_script_cpp.sh; fi
     - $TRAVIS_BUILD_DIR/ci/travis_build_parquet_cpp.sh
@@ -81,7 +82,8 @@ matrix:
     - ARROW_BUILD_WARNING_LEVEL=CHECKIN
     before_script:
     - if [ $ARROW_CI_PYTHON_AFFECTED != "1" ]; then exit; fi
-    - if [ $ARROW_CI_CPP_AFFECTED == "1" ]; then travis_wait 50 $TRAVIS_BUILD_DIR/ci/travis_before_script_cpp.sh; fi
+    # If either C++ or Python changed, we must install the C++ libraries
+    - travis_wait 50 $TRAVIS_BUILD_DIR/ci/travis_before_script_cpp.sh
     script:
     - if [ $ARROW_CI_CPP_AFFECTED == "1" ]; then $TRAVIS_BUILD_DIR/ci/travis_script_cpp.sh; fi
     - $TRAVIS_BUILD_DIR/ci/travis_build_parquet_cpp.sh

ci/travis_build_parquet_cpp.sh

@@ -25,6 +25,7 @@ source $TRAVIS_BUILD_DIR/ci/travis_install_toolchain.sh
 export PARQUET_ARROW_VERSION=$(git rev-parse HEAD)
 
 export PARQUET_BUILD_TOOLCHAIN=$CPP_TOOLCHAIN
+export ARROW_HOME=$ARROW_CPP_INSTALL
 
 PARQUET_DIR=$TRAVIS_BUILD_DIR/parquet
 mkdir -p $PARQUET_DIR