gh-435 pthreads not automatically linked on some systems
gh-435 pthreads not automatically linked on some systems

On some systems (BSD was known and handled already but it's been reported elsewhere too)
cmake/g++ does not automatically link against the pthread library, and build failures can
result.

Use cmake FindThreads module to handle this in a reasonably portable fashion.

Fixes gh-435

Signed-off-by: Richard Chapman <5900d56196fe29dba8e3ae556d57eae28e9f0195@hpccsystems.com>

afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform