Detect logrotate not installed in /usr/sbin

Gentoo started installing the logrotate binary to bin, see
gentoo/gentoo@bc23def
and https://bugs.gentoo.org/show_bug.cgi?id=588156
There might be other systems doing that, which makes it a good idea
to stop hardcoding the path.

fixes #12535

Signed-off-by: Gunnar Beutner <gunnar.beutner@netways.de>

CMakeLists.txt

@@ -99,7 +99,8 @@ endif()
 
 if(NOT DEFINED LOGROTATE_HAS_SU)
   set(LOGROTATE_HAS_SU OFF)
-  execute_process(COMMAND "/usr/sbin/logrotate" ERROR_VARIABLE LOGROTATE_OUTPUT)
+  find_program(LOGROTATE_BINARY logrotate)
+  execute_process(COMMAND ${LOGROTATE_BINARY} ERROR_VARIABLE LOGROTATE_OUTPUT)
   if(LOGROTATE_OUTPUT)
     string(REGEX REPLACE "^logrotate ([0-9.]*).*" "\\1" LOGROTATE_VERSION
       ${LOGROTATE_OUTPUT})