HPCC-8186 Generate precompiled header at eclccserver startup time

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4059 from richardkchapman/generate-precompile

HPCC-8186 Generate precompiled header at eclccserver startup time

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>