Add common tde_setup_gcc_visibility macro #17

In many modules built with CMake, a GCC visibility test is performed. In most, a simple test is performed to determine if the system is a UNIX-type. In tdebase, there is a more in-depth test that checks if tdelibs are built with GCC visibility. The comment also states that it would be good to add a test to see if TQt is built with GCC visibility.

In order not to have all these tests in each module, I put all tests into a common cmake module as tde_setup_gcc_visibility macro. Thanks to this, it will now be simple in each module:

if( WITH_GCC_VISIBILITY )
   tde_setup_gcc_visibility( )
endif( )

The tests in tde_setup_gcc_visibility macro are prepared so that the TQt and tdelibs tests are performed only when they are installed / detected. This allows to use macro not only in the usual modules, but also in tdelibs, and in the future in TQt.

LGTM

before we push to cmake, let's try to build up to the base system with the modified macro.

I have successfully completed the tests of building tdebase and tdelibs 😸

Good enough then

Note: Because for TQt is unable to identify by definitions or header files whether the build was performed with GCC visibility, the test checks to see if the private class methods are visible in exported library symbols.