Occasional compile problems with libexpress appear to be an attempt to
compile with an incompletely generated header. Make the dependency
explicit to assist CMake in generating proper build ordering rules.
This is a classic problem - the file copy will begin, the file will be
"present" in some sense on the file system, and build steps that need it
will begin - but they won't find what they actually need because the
copy isn't fully executed. The standard approach for this is to add a
second file which is only created after the first is done, and then make
the target depend on the second file rather than the first.
Clifford Yapp