src/lib/icu: fix iconv() detection when libiconv is installed

src/lib/icu/meson.build

@@ -12,17 +12,23 @@ if is_windows
   icu_sources += 'Win32.cxx'
 endif
 
+iconv_dep = []
 if icu_dep.found()
   icu_sources += [
     'Util.cxx',
     'Init.cxx',
   ]
 elif not get_option('iconv').disabled()
-  have_iconv = compiler.has_function('iconv', prefix : '#include <iconv.h>')
-  conf.set('HAVE_ICONV', have_iconv)
+  # an installed iconv library will make the builtin iconf() unavailable,
+  # so search for the library first and pass it as (possible) dependency
+  iconv_dep = compiler.find_library('libiconv', required: false)
+  have_iconv = compiler.has_function('iconv', 
+    dependencies: iconv_dep, 
+    prefix : '#include <iconv.h>')
   if not have_iconv and get_option('iconv').enabled()
     error('iconv() not available')
   endif
+  conf.set('HAVE_ICONV', have_iconv)
 endif
 
 icu = static_library(
@@ -31,6 +37,7 @@ icu = static_library(
   include_directories: inc,
   dependencies: [
     icu_dep,
+    iconv_dep,
     fmt_dep,
   ],
 )