Fix cross-compilation for systemd files

Upstreamed from https://salsa.debian.org/mariadb-team/mariadb-10.4/-/blob/master/debian/patches/930314-cross-build.patch
which has been running in Debian successfully for a year now.

Original bug report:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=930314

cmake/systemd.cmake

@@ -17,6 +17,9 @@ MACRO(CHECK_SYSTEMD)
   IF(UNIX)
     INCLUDE(FindPkgConfig)
     # http://www.cmake.org/cmake/help/v3.0/module/FindPkgConfig.html
+    INCLUDE(CheckFunctionExists)
+    INCLUDE(CheckIncludeFiles)
+    INCLUDE(CheckLibraryExists)
     SET(WITH_SYSTEMD "auto" CACHE STRING "Enable systemd scripts and notification support")
     IF(WITH_SYSTEMD STREQUAL "yes" OR WITH_SYSTEMD STREQUAL "auto")
       IF(PKG_CONFIG_FOUND)