With autotools the schemas were compiled into each test suite directory
and the tests set GSETTINGS_SCHEMA_DIR to the test build directory.
With meson's gnome.compile_schemas() we can not define a target directory
so just make sure it is built in the gtk directory and set GSETTINGS_SCHEMA_DIR
to the gtk build directory when running the tests.
This makes the gtk+:gtk suite pass when no gtk is installed on the system.
Visual Studio does not support things like -Wl,export-dynamic, so we
need to export those symbols by using __declspec(dllexport). So, we
decorate these with macros which we define accordingly for this purpose.
https://bugzilla.gnome.org/show_bug.cgi?id=785210
The `-export-dynamic` flag is a libtool-specific flag; since we're not
using libtool with Meson, we should instruct the C compiler to use the
appropriate linker flag instead.
