Add CMake installation target (inspired on PR #30)

CMakeLists.txt

@@ -21,6 +21,8 @@ option( OPTIONAL_LITE_OPT_SELECT_NONSTD  "Select nonstd::optional" OFF )
 
 project( optional_lite LANGUAGES CXX )
 
+include( GNUInstallDirs )
+
 set( package_name "optional-lite" )
 set( include_source_dir "${PROJECT_SOURCE_DIR}/include" )
 
@@ -32,6 +34,13 @@ add_library(
 target_include_directories(
     ${package_name} INTERFACE "$<BUILD_INTERFACE:${include_source_dir}>" )
 
+# Installation:
+
+install( 
+    DIRECTORY ${CMAKE_SOURCE_DIR}/include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR} )
+
+# If requested, build and perform tests, build examples:
+
 enable_testing()
 
 if ( OPTIONAL_LITE_OPT_BUILD_TESTS )