[CMake] Add gRPC_USE_SYSTEMD option

Allows to disable systemd support,
as well as linking with libsystemd,
when it is not required.

The option has three possible values:
AUTO - Default, Will try to find libsystemd.
  If found, systemd support will be enabled.
ON - Enable systemd support. Build will fail
  if libsystemd is not found.
OFF - Disable systemd support.

cmake/systemd.cmake

@@ -12,9 +12,20 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-find_package(systemd)
-if(TARGET systemd)
+set(gRPC_USE_SYSTEMD "AUTO" CACHE STRING "Build with libsystemd support if available. Can be ON, OFF or AUTO")
+
+if (NOT gRPC_USE_SYSTEMD STREQUAL "OFF")
+  if (gRPC_USE_SYSTEMD STREQUAL "ON")
+    find_package(systemd REQUIRED)
+  elseif (gRPC_USE_SYSTEMD STREQUAL "AUTO")
+    find_package(systemd)
+  else()
+    message(FATAL_ERROR "Unknown value for gRPC_USE_SYSTEMD = ${gRPC_USE_SYSTEMD}")
+  endif()
+
+  if(TARGET systemd)
     set(_gRPC_SYSTEMD_LIBRARIES systemd ${SYSTEMD_LINK_LIBRARIES})
     add_definitions(-DHAVE_LIBSYSTEMD)
+  endif()
+  set(_gRPC_FIND_SYSTEMD "if(NOT systemd_FOUND)\n  find_package(systemd)\nendif()")
 endif()
-set(_gRPC_FIND_SYSTEMD "if(NOT systemd_FOUND)\n  find_package(systemd)\nendif()")