ThirdParty: Add optional-lite polyfill

Thanks to all who have made contributions to
https://github.com/martinmoene/optional-lite

third-party/optional-lite/cmake/optional-lite-config-version.cmake.in

@@ -0,0 +1,24 @@
+# Adapted from write_basic_package_version_file(... COMPATIBILITY SameMajorVersion) output
+# ARCH_INDEPENDENT is only present in cmake 3.14 and onwards
+
+set( PACKAGE_VERSION "@package_version@" )
+
+if( PACKAGE_VERSION VERSION_LESS PACKAGE_FIND_VERSION )
+    set( PACKAGE_VERSION_COMPATIBLE FALSE )
+else()
+    if( "@package_version@" MATCHES "^([0-9]+)\\." )
+        set( CVF_VERSION_MAJOR "${CMAKE_MATCH_1}" )
+    else()
+        set( CVF_VERSION_MAJOR "@package_version@" )
+    endif()
+
+    if( PACKAGE_FIND_VERSION_MAJOR STREQUAL CVF_VERSION_MAJOR )
+        set( PACKAGE_VERSION_COMPATIBLE TRUE )
+    else()
+        set( PACKAGE_VERSION_COMPATIBLE FALSE )
+    endif()
+
+    if( PACKAGE_FIND_VERSION STREQUAL PACKAGE_VERSION )
+        set( PACKAGE_VERSION_EXACT TRUE )
+    endif()
+endif()

third-party/optional-lite/cmake/optional-lite-config.cmake.in

@@ -0,0 +1,7 @@
+@PACKAGE_INIT@
+
+# Only include targets once:
+
+if( NOT TARGET @package_name@::@package_name@ )
+    include( "${CMAKE_CURRENT_LIST_DIR}/@package_target@.cmake" )
+endif()