cmake: produce civetweb.h, again

cmake: produce civetweb.h, again

The recent change to do this logic with file copy (and in src/rgw)
resolved the build problem, but now updates to the civetweb
submodule were not reflected in the build.

Move the copy into a custom target which will always source the
current submodule version at build time.

Avoid using the BYPRODUCTS option, as it is not supported in many
older cmake versions (e.g., Centos 7).

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>