Fixed undefined reference to 'clock_gettime' by linking rt library

[francoisbertelatschrodinger]

On CentOS6.8 with a local build of g++ 6.4.0 (the default g++ version on CentOS6.8 is 4.4.7), the build fails with undefined reference to 'clock_gettime'.

Linking Hologram against the rt library and adding rt to SPIRV_TOOLS_LIBRARIES fixed the issue.

This fix is similar to KhronosGroup/SPIRV-Tools#2409

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[TonyBarbour]

#217 attempts to address this problem a little more conservatively

[TonyBarbour]

#217 addresses this problem