pulseaudio: detect intl and iconv through meson dependency objects

pulseaudio: detect intl and iconv through meson dependency objects

The build probes for dgettext and iconv_open with bare symbol checks,
which succeed against libc even when a standalone GNU libintl or
libiconv is on the include path. Their headers redirect the calls to
libintl_* and libiconv_* symbols, so the objects then need -lintl,
which the Makefile supplied through TARGET_LDFLAGS for every link,
and the standalone libiconv was bypassed through LIBICONV_PLUG.

Add the two patches from upstream merge request 877, which switch the
probes to meson's intl and iconv dependency objects. These compile a
test including the header, so they fall back to linking the library
exactly when the header requires it. With BUILD_NLS this links
libintl-full and libiconv-full where they are used; without it, the
libc implementations are used as before. The second patch makes the
libm link conditional in the same way. Drop the -lintl workaround.

Patches taken from merge request currently under review upstream:
https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/877

Signed-off-by: Mirko Vogt <redacted>