Fix GCC visibility, use common tests in CMake rules #1

In Akode I added the usual option WITH_GCC_VISIBILITY and the new common test tde_setup_gcc_visibility. The previous code only set __KDE_HAVE_GCC_VISIBILITY but did not set the compiler options at all. GCC visibility has therefore not been used properly. That's why I checked and added the use of AKODE_EXPORT to the exported functions in each plug-in.

Please test it.

Note: For test you need to build with WITH_ALL_OPTIONS or WITH_GCC_VISIBILITY.