Summary:
in Linux environment, `find_package(gflass)` searches `Findgflags.cmake`.
And it cannot find  https://github.com/facebook/folly/blob/master/CMake/FindGFlags.cmake , because linux is case-sensitive environment in filenames and cause error.

```
CMake Error at CMake/folly-deps.cmake:34 (message):
  Unable to determine the target name for the GFlags package.
Call Stack (most recent call first):
  CMakeLists.txt:65 (include)
```

For linux programmers, this line should find `FindGFlags.cmake` file which exists in the same directory.
Closes https://github.com/facebook/folly/pull/805

Reviewed By: yfeldblum

Differential Revision: D7505908

Pulled By: Orvid

fbshipit-source-id: c30097a25569d293df9a1bfecbee84cece260306