Fix Travis CI build for linux

[bitnot]

Follow up on 99bcd4d Sets the LIBDIR variable to lib.

Failing build Passing build

[bitnot]

@aetilius, @G10h4ck could you check if this makes sense?

Another note: it seems that Travis is having some trouble running the builds from the main repo:

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[aetilius]

Thanks @bitnot. I believe the travis issue is fixed now, are you able to confirm?

[G10h4ck]

The latest change i committed and is already merged in master should fix this already more automatically so this extra change should be not necessary

[bitnot]

Thanks @bitnot. I believe the travis issue is fixed now, are you able to confirm?

Hey @aetilius. Builds seems to be working now - see #30

~I think you would have to push a new commit to make sure Travis CI can start the build.~ ~You can see the past builds failed to start here: https://app.travis-ci.com/github/aetilius/pHash/requests~

~And just to make sure we are on the same page, the issue I am referring to is "Could not authorize build request for aetilius/pHash". You might want to check how you Travis CI plan is configured.~

The latest change i committed and is already merged in master should fix this already more automatically so this extra change should be not necessary

@G10h4ck , The purpose of this PR is to fix a very specific environment - Travis CI bionic linux. I am not familiar with CMake or building C on Linux, so forgive me if I am suggesting nonsense, but it seems to me that Travis CI does not initialize the LIBDIR properly.

On top of that there is (was?) an issue in general preventing builds in the original aetilius/pHash to start (see above). It works fine in my fork however.

[G10h4ck]

Ok for me to merge this PR