Work around ARROW-4026 via environment variable for now

Change-Id: I12553245b37ba879c96b99a678aaa478bfb824b2

.travis.yml

@@ -109,6 +109,7 @@ matrix:
     jdk: openjdk8
     env:
     - ARROW_TRAVIS_GANDIVA=1
+    - ARROW_TRAVIS_GANDIVA_TESTS=1
     - ARROW_TRAVIS_OPTIONAL_INSTALL=1
     - ARROW_CPP_BUILD_TARGETS="gandiva"
     # TODO(wesm): Remove this after ARROW-4026
@@ -159,6 +160,7 @@ matrix:
     addons:
     env:
     - ARROW_TRAVIS_GANDIVA=1
+    - ARROW_TRAVIS_GANDIVA_TESTS=1
     - ARROW_TRAVIS_OPTIONAL_INSTALL=1
     - ARROW_CPP_BUILD_TARGETS="gandiva"
     # TODO(wesm): Remove this after ARROW-4026

ci/travis_before_script_cpp.sh

@@ -99,8 +99,8 @@ fi
 
 if [ $ARROW_TRAVIS_GANDIVA == "1" ]; then
   CMAKE_COMMON_FLAGS="$CMAKE_COMMON_FLAGS -DARROW_GANDIVA=ON"
-  if [ $only_library_mode == "no" ]; then
-    CMAKE_COMMON_FLAGS="$CMAKE_COMMON_FLAGS -DARROW_GANDIVA_BUILD_TESTS=ON"
+  if [ $ARROW_TRAVIS_GANDIVA_TESTS == "1" ]; then
+    CMAKE_COMMON_FLAGS="$CMAKE_COMMON_FLAGS -DARROW_BUILD_TESTS=ON"
   fi
 fi