contrib: medialibrary: link with -latomic

medialibrary uses atomics which may require libatomic depending on the
ABI. This is currently the case on Linux RISC-V. In principles, it
should be pulled as needed by `-pthread` which should be in LDFLAGS.
However:
1) The VLC build system passes `-pthread` in LIBADD instead of LDFLAGS.
2) GNU/libtool discards `-pthread` for CXX-tagged binaries.

This is essentially the same problem as with libupnp lacking
`-lpthread`. In this particular case, medialibrary.pc forces `-lpthread`
explicitly but lacks `-latomic`.

Fixes #26521.

contrib/src/medialibrary/rules.mak

@@ -22,6 +22,13 @@ medialibrary: medialibrary-$(MEDIALIBRARY_VERSION).tar.xz .sum-medialibrary
 
 .medialibrary: medialibrary
 	cd $< && $(HOSTVARS_MESON) $(MESON) -Dlibvlc=disabled build
+	ninja -C $</build
+ifdef HAVE_LINUX
+ifndef HAVE_ANDROID
+	sed -e 's,^Libs\(.*\)-pthread,Libs\1-pthread -latomic,' \
+		-i $</build/meson-private/medialibrary.pc
+endif
+endif
 	cd $< && cd build && ninja install
 	touch $@