libxml: Simplify autoconf macro.
* gnulib-local/m4/libxml.m4 (gl_LIBXML): Don't test for dlopen and shlload.
The test for dlopen failed, because it would require linking with '-ldl'.
The test for shlload failed, because the function on HP-UX is called 'shl_load'
and it would require linking with '-ldld'.
