build: add --enable-smartcard=auto support
build: add --enable-smartcard=auto support

Currently, when running configure with no arguments, smartcard
support is enabled by default, and configure will fail if it cannot
find libcacard. This commit adds a --enable-smartcard=auto mode to
configure which will use automatically enable smartcard support if
libcacard is available, but it will be silently disabled if libcacard
is not available. Passing --enable-smartcard will fail if libcacard
is not available. Passing --disable-smartcard will always disable
smartcard support and will not test for libcacard availability.

elmarco/spice-gtk,fgouget/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,dezelin/spice-gtk,dezelin/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,guodong/spice-gtk,mathslinux/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,flexVDI/spice-gtk,dezelin/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,dezelin/spice-gtk,mathslinux/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,mathslinux/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,mathslinux/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk