Backport of 7ee64d795b6adc0493c1aae68858ccc1142b7a8e

- Collect all "convenience" libraries in
  ${CMAKE_BINARY_DIR}/archive_output_directory

- Extend the ADD_CONVENIENCE_LIBRARY macro with a POST_BUILD target
  which will save library location for later merge with other static libraries.

- Rewrite all the merge targets, with separate rules for
  MSVC / APPLE / LINUX / UNIX

Additional functionality:

- Retry linking on windows if CMAKE_LINKER fails. This handles possible
  change of toolchain from 32bit to 64bit when running cmake, and then
  building with devenv.

Additional for 5.7 only, because of embedded server:

- Add STATIC plugins to known convenience libraries, they may be
  merged into the embedded server library.

- Add ndbclient_static as convenience library, it is needed for embedded.