Specify full shared lib filename in cmake file.

Without this, CMake projects that link against 'SDL2_mixer::SDL2_mixer'
won't be able to find the library. Patch based on equivalent patches in
sdl, sdl2-image, sdl2-ttf.

ok rsadowski@

devel/sdl2-mixer/Makefile

@@ -2,7 +2,7 @@ V =		2.8.0
 COMMENT =	SDL2 multi-channel audio mixer library
 DISTNAME =	SDL2_mixer-${V}
 PKGNAME =	sdl2-mixer-${V}
-REVISION =	0
+REVISION =	1
 CATEGORIES =	devel audio
 
 SHARED_LIBS +=  SDL2_mixer		1.1	# 2.8.0
@@ -36,6 +36,9 @@ USE_GMAKE =	Yes
 
 NO_TEST =	Yes
 
+pre-configure:
+	${SUBST_CMD} ${WRKSRC}/sdl2_mixer-config.cmake.in
+
 post-install:
 	${INSTALL_PROGRAM} ${WRKBUILD}/build/.libs/playmus ${PREFIX}/bin/play2mus
 	${INSTALL_PROGRAM} ${WRKBUILD}/build/.libs/playwave ${PREFIX}/bin/play2wave

devel/sdl2-mixer/patches/patch-sdl2_mixer-config_cmake_in

@@ -0,0 +1,12 @@
+Index: sdl2_mixer-config.cmake.in
+--- sdl2_mixer-config.cmake.in.orig
++++ sdl2_mixer-config.cmake.in
+@@ -111,7 +111,7 @@ if(NOT TARGET SDL2_mixer::SDL2_mixer)
+         unset(_sdl2mixer_dll)
+         unset(_sdl2mixer_imp)
+     else()
+-        set(_sdl2mixer_shl "${_sdl2mixer_libdir}/${CMAKE_SHARED_LIBRARY_PREFIX}SDL2_mixer${CMAKE_SHARED_LIBRARY_SUFFIX}")
++        set(_sdl2mixer_shl "${_sdl2mixer_libdir}/${CMAKE_SHARED_LIBRARY_PREFIX}SDL2_mixer${CMAKE_SHARED_LIBRARY_SUFFIX}.${LIBSDL2_mixer_VERSION}")
+         if(EXISTS "${_sdl2mixer_shl}")
+             add_library(SDL2_mixer::SDL2_mixer SHARED IMPORTED)
+             set_target_properties(SDL2_mixer::SDL2_mixer