[CMake] How to include the generated header file?

[Tan, Tom (Shanghai)]

According to the doc,  generate_export_header(somelib) generates a file
in the ${CMAKE_CURRENT_BUILD_DIR} called somelib_export.h.
What's the recommended way to include this   "somelib_export.h".
#include "somelib_export.h" does not work. And
${CMAKE_CURRENT_BUILD_DIR} is empty too.  I am using v2.8.7. The header
file does get generated, only the doc is not very helpful on how to
include it.