build: add INSTALL option to cmake

docs/getting_started/installation_and_usage.md

@@ -231,6 +231,15 @@ dependencies by other means, some modifications to the library's CMake files mig
 
         [cmake natural units]: https://github.com/mpusz/mp-units/releases/tag/v2.5.0
 
+    [`MP_UNITS_INSTALL`](#MP_UNITS_INSTALL){ #MP_UNITS_INSTALL }
+
+    :   [:octicons-tag-24: 2.5.0][cmake install] · :octicons-milestone-24: `ON`/`OFF` (Default: `ON`)
+
+        Creates an installable target. Users may want to turn this off for example when consuming the
+        library via CMake's `add_subdirectory` or similar mechanisms.
+
+        [cmake install]: https://github.com/mpusz/mp-units/releases/tag/v2.5.0
+
 
 ## Installation and reuse
 
@@ -493,6 +502,34 @@ with the following differences:
         [FAQ](faq.md#why-dont-we-have-cmake-options-to-disable-the-building-of-tests-and-examples).
 
 
+    ### CPM or FetchContent (CMake)
+
+    **mp-units** can be consumed via [CPM](https://github.com/cpm-cmake/CPM.cmake) or CMake's built-in
+    [FetchContent](https://cmake.org/cmake/help/latest/module/FetchContent.html) mechanism. An example
+    CPM configuration is shown below.
+
+    ```cmake
+    CPMFindPackage(
+        NAME mp-units
+        VERSION 2.4.0
+        GIT_TAG 6e06eddf205deaf6c2f2f63500c8c70ec220a99f
+        GITHUB_REPOSITORY mpusz/mp-units
+        SOURCE_SUBDIR src
+        SYSTEM YES
+        EXCLUDE_FROM_ALL YES
+        OPTIONS
+            "MP_UNITS_BUILD_AS_SYSTEM_HEADERS ON"
+            "MP_UNITS_INSTALL OFF"
+    )
+    # ...
+    target_link_libraries(<your_target> <PUBLIC|PRIVATE|INTERFACE> mp-units::mp-units)
+    ```
+
+    !!! note
+
+        This is essentially a shortcut way to do what [Copy + CMake](#copy-cmake) mentions above, so the same
+        prerequisites apply.
+
     ### Install
 
     If you don't want to use Conan in your project and just want to install the **mp-units**

src/CMakeLists.txt

@@ -39,9 +39,11 @@ check_libcxx_in_use(${projectPrefix}LIBCXX)
 # project build options
 option(${projectPrefix}BUILD_AS_SYSTEM_HEADERS "Export library as system headers" OFF)
 option(${projectPrefix}BUILD_CXX_MODULES "Add C++ modules to the list of default targets" OFF)
+option(${projectPrefix}INSTALL "Install the library" ON)
 
 message(STATUS "${projectPrefix}BUILD_AS_SYSTEM_HEADERS: ${${projectPrefix}BUILD_AS_SYSTEM_HEADERS}")
 message(STATUS "${projectPrefix}BUILD_CXX_MODULES: ${${projectPrefix}BUILD_CXX_MODULES}")
+message(STATUS "${projectPrefix}INSTALL: ${${projectPrefix}INSTALL}")
 
 if(${projectPrefix}BUILD_AS_SYSTEM_HEADERS)
     set(${projectPrefix}_AS_SYSTEM SYSTEM)
@@ -135,15 +137,17 @@ add_subdirectory(systems)
 # project-wide wrapper
 add_mp_units_module(mp-units mp-units DEPENDENCIES mp-units::core mp-units::systems MODULE_INTERFACE_UNIT mp-units.cpp)
 
-# local build
-export(EXPORT mp-unitsTargets NAMESPACE mp-units::)
-configure_file("mp-unitsConfig.cmake" "." COPYONLY)
-include(CMakePackageConfigHelpers)
-write_basic_package_version_file(mp-unitsConfigVersion.cmake COMPATIBILITY SameMajorVersion)
+if(${projectPrefix}INSTALL)
+    # local build
+    export(EXPORT mp-unitsTargets NAMESPACE mp-units::)
+    configure_file("mp-unitsConfig.cmake" "." COPYONLY)
+    include(CMakePackageConfigHelpers)
+    write_basic_package_version_file(mp-unitsConfigVersion.cmake COMPATIBILITY SameMajorVersion)
 
-# installation
-install(EXPORT mp-unitsTargets DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/mp-units NAMESPACE mp-units::)
+    # installation
+    install(EXPORT mp-unitsTargets DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/mp-units NAMESPACE mp-units::)
 
-install(FILES mp-unitsConfig.cmake ${CMAKE_CURRENT_BINARY_DIR}/mp-unitsConfigVersion.cmake
-        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/mp-units
-)
+    install(FILES mp-unitsConfig.cmake ${CMAKE_CURRENT_BINARY_DIR}/mp-unitsConfigVersion.cmake
+            DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/mp-units
+    )
+endif()