travis-ci: build gtest without elevated privileges

.travis.yml

@@ -9,11 +9,14 @@ addons:
     - libhdf4-dev
 
 install:
-- cd /usr/src/gtest
-- cmake .
-- make install --jobs=$(nproc)
+- mkdir gtest
+- cd gtest
+- cmake /usr/src/gtest
+- make --jobs=$(nproc)
+- cd ..
 
 script:
-- cmake .
+# https://cmake.org/cmake/help/v3.12/module/FindGTest.html
+- GTEST_ROOT="$PWD/gtest" cmake .
 - make --jobs=$(nproc)
 - tests/hdf4cpp-tests