Add FindJournald cmake module

From https://github.com/fluent/fluent-bit/blob/master/cmake/FindJournald.cmake

cmake/modules/FindJournald.cmake

@@ -0,0 +1,45 @@
+# - Try to find Journald library.
+# Once done this will define
+#
+#  JOURNALD_FOUND - system has Journald
+#  JOURNALD_INCLUDE_DIR - the Journald include directory
+#  JOURNALD_LIBRARIES - Link these to use Journald
+#  JOURNALD_DEFINITIONS - Compiler switches required for using Journald
+# Redistribution and use is allowed according to the terms of the BSD license.
+# For details see the accompanying COPYING-CMAKE-SCRIPTS file.
+#
+
+# Copyright (c) 2015 David Edmundson
+#
+
+# use pkg-config to get the directories and then use these values
+# in the FIND_PATH() and FIND_LIBRARY() calls
+find_package(PkgConfig)
+pkg_check_modules(PC_JOURNALD QUIET systemd)
+
+set(JOURNALD_FOUND ${PC_JOURNALD_FOUND})
+set(JOURNALD_DEFINITIONS ${PC_JOURNALD_CFLAGS_OTHER})
+
+find_path(JOURNALD_INCLUDE_DIR NAMES systemd/sd-journal.h
+  PATHS
+  ${PC_JOURNALD_INCLUDEDIR}
+  ${PC_JOURNALD_INCLUDE_DIRS}
+)
+
+find_library(JOURNALD_LIBRARY NAMES systemd
+  PATHS
+  ${PC_JOURNALD_LIBDIR}
+  ${PC_JOURNALD_LIBRARY_DIRS}
+)
+
+set(JOURNALD_LIBRARIES ${JOURNALD_LIBRARY})
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Journald DEFAULT_MSG JOURNALD_LIBRARY JOURNALD_INCLUDE_DIR)
+
+include(FeatureSummary)
+set_package_properties(Journald PROPERTIES URL https://github.com/systemd
+  DESCRIPTION "Systemd logging daemon")
+
+# show the JOURNALD_INCLUDE_DIR and JOURNALD_LIBRARY variables only in the advanced view
+mark_as_advanced(JOURNALD_INCLUDE_DIR JOURNALD_LIBRARY)