travis: set PYTHON_EXECUTABLE

Author: jcarpent

travis_custom/custom_build

@@ -2,7 +2,7 @@
 set -e
 
 # Setup environment variables.
-export CMAKE_ADDITIONAL_OPTIONS=" ${CMAKE_ADDITIONAL_OPTIONS} -DBUILD_BENCHMARK=\"ON\" -DBUILD_UNIT_TESTS=\"ON\" -DCMAKE_CXX_FLAGS=-DBOOST_SYSTEM_NO_DEPRECATED"
+export CMAKE_ADDITIONAL_OPTIONS=" ${CMAKE_ADDITIONAL_OPTIONS} -DBUILD_BENCHMARK=\"ON\" -DBUILD_UNIT_TESTS=\"ON\" -DCMAKE_CXX_FLAGS=-DBOOST_SYSTEM_NO_DEPRECATED -DPYTHON_EXECUTABLE=$(which python)"
 if [[ ";${DO_INSTALL_DOC_EXCEPT_ON_BRANCH};" == *";${CI_BRANCH};"* ]]; then
   export CMAKE_ADDITIONAL_OPTIONS=" ${CMAKE_ADDITIONAL_OPTIONS} -DINSTALL_DOCUMENTATION=\"OFF\""
 else