meson: use find_program for asn1parser

meson.build

@@ -143,10 +143,11 @@ config_header = configure_file(
   output: 'config.h',
   configuration: conf_data)
 
+asn1parser_prog = find_program('asn1Parser')
 spnego_asn1_target = custom_target('spnego_asn1.c',
   input: 'contrib/spnego/spnego.asn1',
   output: 'spnego_asn1.c',
-  command: ['asn1Parser', '-o', '@OUTPUT@', '-n', 'spnego_asn1_conf',
+  command: [asn1parser_prog, '-o', '@OUTPUT@', '-n', 'spnego_asn1_conf',
             '@INPUT@'])
 
 libdsm_sources = [