CMAKE - generate pocomsg.h in src

Merge https://github.com/guruofquality/poco/commit/922ca412848bc8b5fd05cd6903779eac4e7463a7

fix pocomsg.h generation so the target stops rebuilding
The output directory specified by -h should match the RES_SRCS directory.
Changed the RES_SRCS to specify the srcs directory.