use configure_file to set binary path

CMakeLists.txt

@@ -16,5 +16,6 @@ target_link_libraries(milterfrom ${Milter_LIBRARY} ${CMAKE_THREAD_LIBS_INIT})
 
 install(TARGETS milterfrom DESTINATION bin)
 if(NOT WIN32 AND WITH_SYSTEMD)
-	install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/systemd/milterfrom.service DESTINATION ${SYSTEMD_UNITDIR})
+	configure_file(systemd/milterfrom.service.in ${CMAKE_CURRENT_BINARY_DIR}/systemd/milterfrom.service)
+	install(FILES ${CMAKE_CURRENT_BINARY_DIR}/systemd/milterfrom.service DESTINATION ${SYSTEMD_UNITDIR})
 endif()

systemd/milterfrom.service → systemd/milterfrom.service.in

@@ -4,7 +4,7 @@ Description=A milter that enforces equal envelope and header sender
 [Service]
 Type=forking
 PIDFile=/var/run/milterfrom.pid
-ExecStart=/usr/local/bin/milterfrom -d -p /var/run/milterfrom.pid -u milterfrom -g milterfrom -m 002 -s /var/spool/postfix/milterfrom/milterfrom
+ExecStart=@CMAKE_INSTALL_PREFIX@/bin/milterfrom -d -p /var/run/milterfrom.pid -u milterfrom -g milterfrom -m 002 -s /var/spool/postfix/milterfrom/milterfrom
 ExecReload=/bin/kill -HUP $MAINPID
 
 [Install]