cpp: compat: Add regex.h and corresponding cmake checks
cpp: compat: Add tests for headers
cmake: Add xsd-fu status message for dependency determination

This can take a short while and is run several times, so it keeps
the user informed as to what's happening.
cpp: ome-xml: Add gensrc target to generate all sources

This target depends upon all sources and headers generated by
xsd-fu, and so will allow all source generation to be triggered.
cmake: HeaderTest: Build all header tests twice

This is to detect non-inlined methods and functions in headers
which would result in link errors if included more than once
due to multiple definitions of the same symbol occuring.