Add Linux SDK for building window related extension (DEF-3260)

[britzl]

E.g. the user Tomires wants to build save dialogues for Linux, but the docker container doesn't currently contain such libraries.

The user tried something like this to work "g++ gtktest.cpp pkg-config --cflags --libs gtk+-3.0" in order to add the correct cflags and link flags. It seems like a good idea to support.

[ekharkunov]

I think task can be closed because custom dialogs can be implemented via extension-umgui. WDYT @britzl

[britzl]

Yes, or using https://github.com/andsve/def-diags. Although there is this issue when building that extension for Linux: andsve/def-diags#5