Fix installation when using DESTDIR. (#411)

The Debian package build failed after switching to CMake because it uses `DESTDIR` to install under `debian/tmp`.

CMakeLists.txt

@@ -127,9 +127,9 @@ endif()
 set(MAPNIK_FONTS_DIR "${MAPNIK_FONTS_DIR}")
 set(MAPNIK_FONTS_DIR_RECURSE 0)
 set(MAPNIK_PLUGINS_DIR "${MAPNIK_PLUGINS_DIR}")
-set(RENDERD_CONFIG "/${CMAKE_INSTALL_SYSCONFDIR}/renderd.conf")
-set(RENDERD_RUN_DIR "/${CMAKE_INSTALL_RUNSTATEDIR}/renderd")
-set(RENDERD_TILE_DIR "/${CMAKE_INSTALL_LOCALSTATEDIR}/cache/renderd/tiles")
+set(RENDERD_CONFIG "${CMAKE_INSTALL_SYSCONFDIR}/renderd.conf")
+set(RENDERD_RUN_DIR "${CMAKE_INSTALL_RUNSTATEDIR}/renderd")
+set(RENDERD_TILE_DIR "${CMAKE_INSTALL_LOCALSTATEDIR}/cache/renderd/tiles")
 
 set(RENDERD_PIDFILE "${RENDERD_RUN_DIR}/renderd.pid")
 set(RENDERD_SOCKET "${RENDERD_RUN_DIR}/renderd.sock")
@@ -226,7 +226,7 @@ install(
 install(
   FILES
     ${PROJECT_BINARY_DIR}/renderd.conf
-  DESTINATION /${CMAKE_INSTALL_SYSCONFDIR}
+  DESTINATION ${CMAKE_INSTALL_SYSCONFDIR}
 )
 
 # Targets