lib/icu: fix build with iconv()

NEWS

@@ -3,6 +3,7 @@ ver 0.21.16 (not yet released)
   - curl: request the "resourcetype" property to fix database update
 * update
   - fix crash when music_directory is not a directory
+* fix build with iconv() instead of ICU
 
 ver 0.21.15 (2019/09/25)
 * decoder

src/lib/icu/meson.build

@@ -20,7 +20,7 @@ if icu_dep.found()
 elif not get_option('iconv').disabled()
   have_iconv = compiler.has_function('iconv')
   conf.set('HAVE_ICONV', have_iconv)
-  if get_option('iconv').enabled()
+  if not have_iconv and get_option('iconv').enabled()
     error('iconv() not available')
   endif
 endif