file's sole purpose was to provide a dependency on pkg-config and set
some environment variables. Instead, turn pkg-config into a "tool"
in the tools framework, where the pkg-config wrapper automatically
adds PKG_CONFIG_LIBDIR to the environment before invoking the real
pkg-config.
For all package Makefiles that included pkg-config/buildlink3.mk, remove
that inclusion and replace it with USE_TOOLS+=pkg-config.
libglademm 2.6 is API/ABI-compatible with libglademm 2.4.
Changes since libglademm 2.4:
* Glade::Xml: Added connect_clicked() for easy connection of
buttons and menuitems signals.
* Glade::VariablesMap: now handles Entry, ComboTextEntry,
SpinButton, *Scale, Calendar and CheckBox.
