package/imagemagick: fix configure error with --disable-opencl in setups without...

package/imagemagick: fix configure error with --disable-opencl in setups without libltdl

Fixes:
http://autobuild.buildroot.net/results/d9a/d9a84b642357f758c3f84270fb9a109abd7e2684/

configure.ac contains a test using $ax_cv_check_cl_libcl:

if test "$build_modules" != 'no' || test "X$ax_cv_check_cl_libcl" != Xno; then
  AC_MSG_RESULT([-------------------------------------------------------------])
  AC_MSG_CHECKING([for libltdl])

But ax_cv_check_cl_libcl is only assigned a value (yes/no) if
--disable-opencl is NOT passed, as the assignment logic is inside a
conditional:

AC_ARG_ENABLE([opencl],
    [AC_HELP_STRING([--disable-opencl],
                    [do not use OpenCL])],
    [disable_opencl=$enableval],
    [disable_opencl='yes'])

if test "$disable_opencl" = 'yes'; then
  ..
  AC_CACHE_CHECK([for OpenCL library], [ax_cv_check_cl_libcl],

So configure errors out if --disable-opencl is passed on setups where
libltdl isn't available:

checking if libltdl package is complete... no
configure: error: in `/home/naourr/work/instance-0/output-1/build/imagemagick-7.0.8-59':
configure: error: libltdl is required for modules and OpenCL builds

As a workaround, explictly set ax_cv_check_cl_libcl=no to skip this
conditional.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>