When "getopt()" is not available, various of the netcdf-c utilities
use XGetopt instead. This occurs primarily when building under Window,
so the build changes are restricted to CMake.

This PR tries to isolate XGetopt.c to the libdispatch directory
and then builds the various utilities using this cliche:
````
IF(USE_X_GETOPT)
  SET(XGETOPTSRC "${CMAKE_CURRENT_SOURCE_DIR}/../libdispatch/XGetopt.c")
ENDIF()
````

This avoids the need to copy XGetopt.c to all the directories that
use it.