meson: port ncpmc iconv solution

Properly deals with iconv, unlike the current solution. have_iconv fails
when libiconv CFLAGS are passed to the compiler. Tested under OpenWrt
with its CONFIG_BUILD_NLS, which adds libiconv include flags.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

NEWS

@@ -1,6 +1,7 @@
 ver 0.23.9 (not yet released)
 * output
   - pipewire: set app icon
+* improve iconv detection
 
 ver 0.23.8 (2022/07/09)
 * storage

src/lib/icu/meson.build

@@ -18,18 +18,26 @@ if icu_dep.found()
     'Util.cxx',
     'Init.cxx',
   ]
+else
+  if meson.version().version_compare('>= 0.60')
+    iconv_dep = dependency('iconv', required: get_option('iconv'))
+    conf.set('HAVE_ICONV', iconv_dep.found())
   elif not get_option('iconv').disabled()
-  # an installed iconv library will make the builtin iconv() unavailable,
+    iconv_open_snippet = '''#include <iconv.h>
-  # so search for the library first and pass it as (possible) dependency
+      int main() {
-  iconv_dep = compiler.find_library('libiconv', required: false)
+      iconv_open("","");
-  have_iconv = compiler.has_function('iconv', 
+      }'''
-    dependencies: iconv_dep, 
+    have_iconv = compiler.links(iconv_open_snippet, name: 'iconv_open')
-    prefix : '#include <iconv.h>')
+    if not have_iconv
+      iconv_dep = compiler.find_library('iconv', required: false)
+      have_iconv = compiler.links(iconv_open_snippet, dependencies: iconv_dep, name: 'iconv_open')
+    endif
     if not have_iconv and get_option('iconv').enabled()
       error('iconv() not available')
     endif
     conf.set('HAVE_ICONV', have_iconv)
   endif
+endif
 
 icu = static_library(
   'icu',