meson: use best practices lookup for iconv support

Meson 0.60.0 adds support for an "iconv" dependency that always does the
right thing for you, so that you don't need to think about it. It also
has some side advantages:
- you can do dependency fallback, and --force-fallback-for=iconv works
- it logs one line, not two, and that is "dependency found? yes or no"

Since Aegisub doesn't mandate the use of such new versions of Meson, we
cannot assume the dependency works. Instead, adapt to the version of
Meson being used: on new enough versions of Meson, use the new
dependency, but on older versions of Meson, use the pre-existing logic,
which isn't as nice but has been producing correct results so far.

meson.build

@@ -78,11 +78,15 @@ cc = meson.get_compiler('c')
 deps += cc.find_library('m', required: false)
 deps += cc.find_library('dl', required: false)
 
+if meson.version().version_compare('>=0.60.0')
+  iconv_dep = dependency('iconv', fallback: ['iconv', 'libiconv_dep'])
+else
   iconv_dep = cc.find_library('iconv', required: false)
   if not (iconv_dep.found() or cc.has_function('iconv_open'))
       iconv_sp = subproject('iconv') # this really needs to be replaced with a proper port
       iconv_dep = iconv_sp.get_variable('libiconv_dep')
   endif
+endif
 deps += iconv_dep
 
 deps += dependency('libass', version: '>=0.9.7',