meson: simplify intl lookups using Meson's builtin dependency lookup

intl is complicated to look up. Some of that complexity now resides in
Meson, since 0.59.0, via a `dependency('intl')` lookup, so use that
instead.

The Meson lookup doesn't include all the checks here, but likewise this
meson.build doesn't include all the checks in Meson. Particularly, the
following are different:

- Meson accurately detects support built into libc, even if that
  conflicts with an external library version (which should be detected as
  broken and thus not-found, but glib does not do so).

  The problem here is that depending on which libintl.h header is first
  in the search path, the *gettext symbols may be the libc ABI, or they
  may be renamed to libintl_*gettext, then additionally take over the
  *gettext names via a macro, in order to invoke the external library
  version even on systems where there is a libc builtin. This means that
  checking for `cc.has_function()` correctly reports that there is such
  a function in libc, but that unfortunately does not mean it is usable,
  because source code referencing `ngettext` etc. will expect to be
  linked to `libintl_ngettext`.

- glib checks whether the found intl requires pthread, rather than
  simply trusting the result of `cc.find_library()` for the external
  library case.

Do the heavy lifting by using Meson to check for intl, and select the
correct implementation, but do a post-discovery check if the symbol is
linkable both with/without pthread.

The logic is still a bit hairy, and eventually more of the logic could
be moved into Meson. But it's better than before.

Fixes incorrect detection of intl on musl-based systems (which have a
less capable libc intl), when GNU libintl is installed as an external
library.

meson.build

@@ -2030,42 +2030,37 @@ libz_dep = dependency('zlib')
 # FIXME: glib-gettext.m4 has much more checks to detect broken/uncompatible
 # implementations. This could be extended if issues are found in some platforms.
 libintl_deps = []
-if cc.has_function('ngettext')
-  have_bind_textdomain_codeset = cc.has_function('bind_textdomain_codeset')
-else
-  # First just find the bare library.
-  libintl = cc.find_library('intl', required : false)
-  # The bare library probably won't link without help if it's static.
-  if libintl.found() and not cc.has_function('ngettext', dependencies : libintl)
-     libintl_iconv = cc.find_library('iconv', required : false)
-     # libintl supports different threading APIs, which may not
-     # require additional flags, but it defaults to using pthreads if
-     # found. Meson's "threads" dependency does not allow you to
-     # prefer pthreads. We may not be using pthreads for glib itself
-     # either so just link the library to satisfy libintl rather than
-     # also defining the macros with the -pthread flag.
-     libintl_pthread = cc.find_library('pthread', required : false)
-     # Try linking with just libiconv.
-     if libintl_iconv.found() and cc.has_function('ngettext', dependencies : [libintl, libintl_iconv])
-       libintl_deps += [libintl_iconv]
-     # Then also try linking with pthreads.
-     elif libintl_iconv.found() and libintl_pthread.found() and cc.has_function('ngettext', dependencies : [libintl, libintl_iconv, libintl_pthread])
-       libintl_deps += [libintl_iconv, libintl_pthread]
-     else
-       libintl = disabler()
-     endif
-  endif
-  if not libintl.found()
-    libintl = subproject('proxy-libintl').get_variable('intl_dep')
-    libintl_deps = [libintl] + libintl_deps
-    have_bind_textdomain_codeset = true  # proxy-libintl supports it
+libintl = dependency('intl', required: false)
+if libintl.found()
+  # libintl supports different threading APIs, which may not
+  # require additional flags, but it defaults to using pthreads if
+  # found. Meson's "threads" dependency does not allow you to
+  # prefer pthreads. We may not be using pthreads for glib itself
+  # either so just link the library to satisfy libintl rather than
+  # also defining the macros with the -pthread flag.
+  #
+  # Meson's builtin dependency lookup as of 0.60.0 doesn't check for
+  # pthread, so we do this manually here.
+  if cc.has_function('ngettext', dependencies : libintl)
+    libintl_deps += [libintl]
   else
-    libintl_deps = [libintl] + libintl_deps
-    have_bind_textdomain_codeset = cc.has_function('bind_textdomain_codeset', 
-                                                   dependencies : libintl_deps)
+    libintl_pthread = cc.find_library('pthread', required : false)
+    if libintl_pthread.found() and cc.has_function('ngettext', dependencies : [libintl, libintl_pthread])
+      libintl_deps += [libintl, libintl_pthread]
+    else
+      libintl = disabler()
+    endif
   endif
 endif
 
+if libintl.found()
+  have_bind_textdomain_codeset = cc.has_function('bind_textdomain_codeset', dependencies: libintl_deps)
+else
+  libintl = subproject('proxy-libintl').get_variable('intl_dep')
+  libintl_deps = [libintl]
+  have_bind_textdomain_codeset = true  # proxy-libintl supports it
+endif
+
 glib_conf.set('HAVE_BIND_TEXTDOMAIN_CODESET', have_bind_textdomain_codeset)
 
 # We require gettext to always be present