Add --enable-gnutls option
Add --enable-gnutls option

Previously HAVE_LIBGNUTLS was never set in config.h even if gnutls was
detected as AC_CHECK_LIB default action-if-found was overriden by
enable_gnutls=yes
This patch adds an --enable-gnutls option and will call AC_CHECK_LIB
with the default action to write HAVE_LIBGNUTLS in config.h

Signed-off-by: Fabrice Fontaine <edf03f3512e0539bc34442e6bd5d9ea573f7ba37@orange.com>

federicobriata/lxc,GreatFruitOmsk/lxc,GreatFruitOmsk/lxc,hfuCN/lxc,thmo/lxc,LynxChaus/lxc,federicobriata/lxc,federicobriata/lxc,hallyn/lxc,caglar10ur/lxc,hallyn/lxc,aeris/lxc,ss1h2a3tw/lxc,cjwatson/lxc,hfuCN/lxc,aeris/lxc,cjwatson/lxc,terceiro/lxc,LynxChaus/lxc,ysbnim/lxc,cjwatson/lxc,thmo/lxc,GreatFruitOmsk/lxc,terceiro/lxc,caglar10ur/lxc,ysbnim/lxc,LynxChaus/lxc,ysbnim/lxc,thmo/lxc,terceiro/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,aeris/lxc,hallyn/lxc,hfuCN/lxc,caglar10ur/lxc