CMakeLists.txt require Gtest,Gflags, Threads if building tests

[nmushell]

If you have are going to build tests SNAPPY_BUILD_TESTS you will require gflag and gtest libraries. Let's make them required and move the find_package into the if(SNAPPY_BUILD_TESTS) code block when.

This PR is similar to #62

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[nmushell]

I signed it!

[googlebot]

CLAs look good, thanks!

[cmumford]

@googlebot rescan

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A Googler has manually verified that the CLAs look good.

(Googler, please make sure the reason for overriding the CLA status is clearly documented in these comments.)

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[ghost]

@cmumford , this one looks outdated/obsolete

[pwnall]

Thank you for your contribution!

We're not currently interested in contributions to the build configuration. We now have a paragraph explaining the situation in https://github.com/google/snappy/blob/master/README.md#contributing-to-the-snappy-project