PRIVATE_HEADER
--------------

Specify private header files in a FRAMEWORK shared library target.

Shared library targets marked with the FRAMEWORK property generate
frameworks on OS X and normal shared libraries on other platforms.
This property may be set to a list of header files to be placed in the
PrivateHeaders directory inside the framework folder.  On non-Apple
platforms these headers may be installed using the PRIVATE_HEADER
option to the install(TARGETS) command.