cmake: allow overriding sysusers.d install directory

Make the systemd sysusers.d install directory configurable through the
`SYSUSERS_DIR` CMake cache variable.

The existing logic discovers `sysusersdir` from systemd's pkg-config data,
falling back to `/usr/lib/sysusers.d`. That preserves the right default for
system installs, but it does not let packagers redirect the file into a staged
install prefix.

Packaging systems often build into an isolated prefix or staging directory and
must not install files directly into /usr/lib. Other rdma-core install paths,
such as systemd service and udev rule directories, are already configurable.
Making `SYSUSERS_DIR` configurable follows the same pattern.

Signed-off-by: Peter Andreas Entschev <peter@entschev.com>

Author: pentschev

CMakeLists.txt

@@ -599,11 +599,13 @@ include_directories(${SYSTEMD_INCLUDE_DIRS})
 RDMA_DoFixup("${SYSTEMD_FOUND}" "systemd/sd-daemon.h")
 
 # https://www.freedesktop.org/software/systemd/man/latest/sysusers.d.html
-set(SYSUSERS_DIR "")
-if (NOT ${CMAKE_VERSION} VERSION_LESS "3.4")
-  pkg_check_modules(SYSTEMD_PC systemd QUIET)
-  if (SYSTEMD_PC_FOUND)
-    pkg_get_variable(SYSUSERS_DIR systemd sysusersdir)
+set(SYSUSERS_DIR "" CACHE PATH "Location for systemd sysusers.d files")
+if ("${SYSUSERS_DIR}" STREQUAL "")
+  if (NOT ${CMAKE_VERSION} VERSION_LESS "3.4")
+    pkg_check_modules(SYSTEMD_PC systemd QUIET)
+    if (SYSTEMD_PC_FOUND)
+      pkg_get_variable(SYSUSERS_DIR systemd sysusersdir)
+    endif()
   endif()
 endif()
 if ("${SYSUSERS_DIR}" STREQUAL "")