HPCC-11045 Add options for ECL Watch Build

Allow SKIP, SOURCES and IF_MISSING (only build if missing)

Fixes HPCC-11045

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>
Merge pull request #5554 from GordonSmith/HPCC-11045

HPCC-11045 Add options for ECL Watch Build

Reviewed-By: Xiaoming Wang <xiaoming.wang@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>