gtk3 include handling - Improving IDE support

[Stephan Bergmann]

On 15/12/2018 22:42, Michael Weghorn wrote:
> 1) make sure the include-related compiler flags actually end up in the
> 'INCLUDE' section in the generated JSON file
> 
> As far as I can see, 'gb_Library_set_include' is the way to set the
> '-I'/'-isystem' compiler flags in gbuild, and that approach is used for
> pretty much all other libraries.
> https://gerrit.libreoffice.org/#/c/65207 demonstrates this approach.

Indeed, for better or worse, e.g. all the *_CFLAGS in 
RepositoryExternal.mk are passed into gb_LinkTarget_set_include, not 
gb_LinkTarget_set_cxxflags.  Feel free to go with this approach, I'd say 
in vcl/Library_vclplug_gtk3.mk, I'd say.