configure: Look for the libdwarf headers also in /usr/include/libdwarf
configure: Look for the libdwarf headers also in /usr/include/libdwarf

Before this change, configure script looked for them only in
$includedir, which doesn't work well with jhbuild.

Fixes #471

matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak