Use modern CMake imported target for libgig to ensure proper RPATH propagation.

The old-style link_directories()/link_libraries() pattern does not propagate
library paths to the install RPATH, causing libgig.so to not be found at
runtime when it is installed in a non-standard directory (e.g. lib/libgig/).

Using IMPORTED_TARGET with pkg_check_modules creates a PkgConfig::GIG target
that carries include dirs, library paths, and link flags, and CMake correctly
propagates these to the install RPATH via target_link_libraries().

Also set CMAKE_INSTALL_RPATH_USE_LINK_PATH in the GigPlayer scope, since the
upstream code only sets it in src/CMakeLists.txt which does not apply to the
sibling plugins/ directory.

diff -ruN a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -550,7 +550,7 @@
 
 # check for libgig
 If(WANT_GIG)
-	PKG_CHECK_MODULES(GIG gig)
+	PKG_CHECK_MODULES(GIG IMPORTED_TARGET gig)
 	IF(GIG_FOUND)
 		SET(LMMS_HAVE_GIG TRUE)
 		SET(STATUS_GIG "OK")
diff -ruN a/plugins/GigPlayer/CMakeLists.txt b/plugins/GigPlayer/CMakeLists.txt
--- a/plugins/GigPlayer/CMakeLists.txt
+++ b/plugins/GigPlayer/CMakeLists.txt
@@ -1,17 +1,16 @@
 if(LMMS_HAVE_GIG)
 	INCLUDE(BuildPlugin)
-	include_directories(SYSTEM ${GIG_INCLUDE_DIRS})
 	SET(CMAKE_AUTOUIC ON)
+	SET(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
 
 	# Required for not crashing loading files with libgig
 	add_compile_options("-fexceptions")
 
-	link_directories(${GIG_LIBRARY_DIRS})
-	link_libraries(${GIG_LIBRARIES})
 	build_plugin(gigplayer
 		GigPlayer.cpp GigPlayer.h PatchesDialog.cpp PatchesDialog.h PatchesDialog.ui
 		MOCFILES GigPlayer.h PatchesDialog.h
 		EMBEDDED_RESOURCES "${CMAKE_CURRENT_SOURCE_DIR}/*.png"
 	)
+	target_link_libraries(gigplayer PkgConfig::GIG)
 	target_link_libraries(gigplayer SampleRate::samplerate)
 endif(LMMS_HAVE_GIG)