Improve dependencies checks in the build process

- Rename all Find*.cmake files to uppercase, to make things more consistent,
  and fix some issues with them (notably the REQUIRED flag)

- Rename SYSTEMDDAEMON to SYSTEMD and only use the libsystemd instead of
  libsystemd-daemon because it's deprecated for a long time now

- Provide a WITH_* and WITHOUT_* switch for all optional dependencies

- Document things in the INSTALL file

cmake/Modules/FindSYSTEMD.cmake

@@ -0,0 +1,39 @@
+# - Find SystemdDaemon
+# Find the systemd daemon library
+#
+# This module defines the following variables:
+#   SYSTEMD_FOUND  -  True if library and include directory are found
+# If set to TRUE, the following are also defined:
+#   SYSTEMD_INCLUDE_DIRS  -  The directory where to find the header file
+#   SYSTEMD_LIBRARIES  -  Where to find the library file
+#
+# For conveniance, these variables are also set. They have the same values
+# than the variables above.  The user can thus choose his/her prefered way
+# to write them.
+#   SYSTEMD_LIBRARY
+#   SYSTEMD_INCLUDE_DIR
+#
+# This file is in the public domain
+
+include(FindPkgConfig)
+pkg_check_modules(SYSTEMD libsystemd)
+
+if(NOT SYSTEMD_FOUND)
+  find_path(SYSTEMD_INCLUDE_DIRS NAMES systemd/sd-daemon.h
+    DOC "The Systemd include directory")
+
+  find_library(SYSTEMD_LIBRARIES NAMES systemd
+    DOC "The Systemd library")
+
+  # Use some standard module to handle the QUIETLY and REQUIRED arguments, and
+  # set SYSTEMD_FOUND to TRUE if these two variables are set.
+  include(FindPackageHandleStandardArgs)
+  find_package_handle_standard_args(SYSTEMD REQUIRED_VARS SYSTEMD_LIBRARIES SYSTEMD_INCLUDE_DIRS)
+
+  if(SYSTEMD_FOUND)
+    set(SYSTEMD_LIBRARY ${SYSTEMD_LIBRARIES})
+    set(SYSTEMD_INCLUDE_DIR ${SYSTEMD_INCLUDE_DIRS})
+  endif()
+endif()
+
+mark_as_advanced(SYSTEMD_INCLUDE_DIRS SYSTEMD_LIBRARIES)