contrib: gme: Add libm to pkg-config file

This library links against libm, so it should be added into its
pkg-config file for static builds.

Refs #27262

contrib/src/gme/add-libm.patch

+--- game-music-emu-0.6.3/gme/libgme.pc.in.old	2022-08-26 08:35:03.000000000 +0200
++++ game-music-emu-0.6.3/gme/libgme.pc.in	2022-08-26 08:35:18.000000000 +0200
+@@ -13,4 +13,4 @@
+ Version: @GME_VERSION@
+ Cflags: -I${includedir}
+ Libs: -L${libdir} -lgme
+-Libs.private: -lstdc++ @PKG_CONFIG_ZLIB@
++Libs.private: -lstdc++ -lm @PKG_CONFIG_ZLIB@

contrib/src/gme/rules.mak

@@ -15,6 +15,8 @@ DEPS_gme = zlib $(DEPS_zlib)
 game-music-emu: game-music-emu-$(GME_VERSION).tar.xz .sum-gme
 	$(UNPACK)
 	$(APPLY) $(SRC)/gme/skip-underrun.patch
+	$(APPLY) $(SRC)/gme/add-libm.patch
+	$(call pkg_static,"gme/libgme.pc.in")
 	$(MOVE)
 
 .gme: game-music-emu toolchain.cmake