Use relative paths for installation

The CMake documentation [1] advises against using `CMAKE_INSTALL_FULL_*`
in `install()` commands.

[1] https://cmake.org/cmake/help/v3.30/module/GNUInstallDirs.html#result-variables

src/sdl/CMakeLists.txt

@@ -71,15 +71,15 @@ endif()
 # Installation scripts.
 install(
     PROGRAMS ${PROJECT_BINARY_DIR}/vbam${CMAKE_EXECUTABLE_SUFFIX}
-    DESTINATION ${CMAKE_INSTALL_FULL_BINDIR}
+    DESTINATION ${CMAKE_INSTALL_BINDIR}
 )
 install(
     FILES ${CMAKE_CURRENT_LIST_DIR}/vbam.cfg-example
-    DESTINATION ${CMAKE_INSTALL_FULL_SYSCONFDIR}
+    DESTINATION ${CMAKE_INSTALL_SYSCONFDIR}
     RENAME vbam.cfg
 )
 
 if(UNIX)
     # man pages.
-    install(FILES ${CMAKE_SOURCE_DIR}/src/debian/vbam.6 DESTINATION ${CMAKE_INSTALL_FULL_MANDIR}/man6)
+    install(FILES ${CMAKE_SOURCE_DIR}/src/debian/vbam.6 DESTINATION ${CMAKE_INSTALL_MANDIR}/man6)
 endif()

src/wx/CMakeLists.txt

@@ -1010,13 +1010,13 @@ install(
 # Installation scripts.
 install(
     PROGRAMS ${PROJECT_BINARY_DIR}/visualboyadvance-m${CMAKE_EXECUTABLE_SUFFIX}
-    DESTINATION ${CMAKE_INSTALL_FULL_BINDIR}
+    DESTINATION ${CMAKE_INSTALL_BINDIR}
 )
 install(
     FILES ${CMAKE_SOURCE_DIR}/src/vba-over.ini
-    DESTINATION ${CMAKE_INSTALL_FULL_DATADIR}/vbam
+    DESTINATION ${CMAKE_INSTALL_DATADIR}/vbam
 )
 
 if (UNIX)
-    install(FILES ${CMAKE_SOURCE_DIR}/src/debian/visualboyadvance-m.6 DESTINATION ${CMAKE_INSTALL_FULL_MANDIR}/man6)
+    install(FILES ${CMAKE_SOURCE_DIR}/src/debian/visualboyadvance-m.6 DESTINATION ${CMAKE_INSTALL_MANDIR}/man6)
 endif()