Fix static linking with mingw

Fix static linking with mingw

Now that we're normally linking libdbus-1 dynamically, we need to
use DBUS_STATIC_BUILD_CPPFLAGS in every Makefile that would normally
link it dynamically, but might link it statically if we are only
building static libraries.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=83115
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>