configure: Fix build if cheese is explicitly disabled

If --with-cheese=no is passed, the BUILD_CHEESE conditional is never
defined and the build breaks - move the call to AM_CONDITIONAL to
define it in any case.

configure.ac

@@ -164,14 +164,16 @@ AC_ARG_WITH([cheese],
 
 if test x"$with_cheese" != x"no" ; then
    PKG_CHECK_MODULES(CHEESE, gstreamer-0.10 cheese-gtk >= 2.91.5, [have_cheese=yes], [have_cheese=no])
-   AM_CONDITIONAL(BUILD_CHEESE, test x${have_cheese} = xyes)
    if test x${have_cheese} = xyes; then
      AC_DEFINE(HAVE_CHEESE, 1, [Define to 1 to enable cheese webcam support])
    fi
    if test x${with_cheese} = xyes && test x${have_cheese} = xno; then
      AC_MSG_ERROR([Cheese configured but not found])
    fi
+else
+   have_cheese=no
 fi
+AM_CONDITIONAL(BUILD_CHEESE, test x${have_cheese} = xyes)
 
 # This is a hard-dependency for the region and user-accounts panels
 PKG_CHECK_MODULES(ISOCODES, iso-codes)