add option for -fPIC compiler flag when building a static library on Linux

[nkowdley]

Currently, I am trying to build a Shared Library which will link against a static version of this sdk.

I keep getting an error on using the -fpic compiler flag, exactly like this issue aws/aws-sdk-cpp#77.

Is there a reason why this hasn't been added as a CMake Option for this SDK? If there isn't, I can make a MR to add this in.

[vareddy-zz]

HI @nkowdley,
Thank you for your interest in the AWS IoT SDK.
This is a valid fix. If you could make a pull request, we will test the changes internally and merge it after we are done.
Thanks!
Varun