Define GEANY_{EXPORT,API}_SYMBOL from the build system

This makes it easier to define it consistently to what the compiler
and platform supports, and avoids having to include a special header
everywhere, which is some kind of a problem for separate libraries
like TagManager and especially Scintilla.

As we only use these macros from the source and not the headers, it
is fine for it to be defined to a configure-time check from the build
system.

Warning: Although Waf and Windows makefiles are updated they are not
         tested an will probably required tuning.
Fix exporting Scintilla symbols