Cache build directory

.travis.yml

@@ -67,6 +67,10 @@ matrix:
 #    script: export OS=osx; export COMPILER='clang-default'; export STL='libc++'; extras/scripts/postsubmit.sh
 #      DebugPlain
 
+cache:
+  directories:
+  - build
+
 services:
 - docker
 sudo: required

travis/postsubmit-helper.sh

@@ -92,8 +92,8 @@ ReleasePlain)         CMAKE_ARGS=(-DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_FLAGS="
 *) echo "Error: you need to specify one of the supported postsubmit modes (see postsubmit.sh)."; exit 1 ;;
 esac
 
-rm -rf build
-mkdir build
+#rm -rf build
+mkdir -p build
 cd build
 cmake .. "${CMAKE_ARGS[@]}"
 echo