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.
The flags in this variables are used to tune the linker behavior on the
final libgeany (currently set the version information), so should only
used on really linked libraries, not Libtool helper libraries.
This prevents having to export those callbacks and put them in the
global namespace. Also, use inline shell script in Makefile.am instead
of a Python script which should be more portable (by default) and gets
rid of the helper script.
Checks if the compiler supports -fvisibility and the linker supports
-dynamic-list arguments and use them instead of hardcoding. The new
geany-lib.m4 also accomodates future use of Libtool versioning.
