meson: Fix detection of a system-provided proxy-libintl
proxy-libintl defines ngettext() as a define in the header that points to the actual symbol in the library which is g_libintl_ngettext(). Same with bind_textdomain_codeset().
cc @nirbheek
This patch has been used in GStreamer cerbero build system for a while and has been tested with native MSVC Windows and mingw cross-builds, Android, iOS, macOS and Linux.
I don't remember what it fixed exactly, I suspect a build issue on Android, iOS or macOS.