Fix lists of paths in posix environment (#7034)

Empty paths in a colon separated list would be considered as the current
directory, so have to ensure $PATH and $LD_LIBRARY_PATH are not empty
before separating it with :

.travis.yml

@@ -30,7 +30,7 @@ before_script:
   - sh build.sh
   - cd ..
   - sed -i -e 's,cc = gcc,cc = clang,' config/nim.cfg
-  - export PATH=$(pwd)/bin:$PATH
+  - export PATH=$(pwd)/bin${PATH:+:$PATH}
 script:
   - nim c koch
   - ./koch boot