CMake: Optional Install if Embedded

When adding this library as embedded library with private
"target link", e.g. only used inside private source files, the
library does not need to be installed when the main project gets
installed.

This adds an additional option `JSON_Install` similar to the
test-build control switch in order to skip installing headers and
CMake config files if requested.

Avoids using
```cmake
add_subdirectory(path/to/nlohmann_json EXCLUDE_FROM_ALL)
```

which has further side-effects:
https://cmake.org/cmake/help/v3.0/command/add_subdirectory.html

CMakeLists.txt

@@ -16,6 +16,7 @@ include(ExternalProject)
 ## OPTIONS
 ##
 option(JSON_BuildTests "Build the unit tests when BUILD_TESTING is enabled." ON)
+option(JSON_Install "Install CMake targets during install step." ON)
 option(JSON_MultipleHeaders "Use non-amalgamated version of the library." OFF)
 
 ##
@@ -92,32 +93,34 @@ configure_file(
     @ONLY
 )
 
-install(
-    DIRECTORY ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}
-    DESTINATION ${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}
-)
-install(
-    FILES ${NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE} ${NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE}
-    DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
-)
-if (NLOHMANN_ADD_NATVIS)
+if(JSON_Install)
     install(
-        FILES ${NLOHMANN_NATVIS_FILE}
-        DESTINATION .
-)
+        DIRECTORY ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}
+        DESTINATION ${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}
+    )
+    install(
+        FILES ${NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE} ${NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE}
+        DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
+    )
+    if (NLOHMANN_ADD_NATVIS)
+        install(
+            FILES ${NLOHMANN_NATVIS_FILE}
+            DESTINATION .
+    )
+    endif()
+    export(
+        TARGETS ${NLOHMANN_JSON_TARGET_NAME}
+        NAMESPACE ${PROJECT_NAME}::
+        FILE ${NLOHMANN_JSON_CMAKE_PROJECT_TARGETS_FILE}
+    )
+    install(
+        TARGETS ${NLOHMANN_JSON_TARGET_NAME}
+        EXPORT ${NLOHMANN_JSON_TARGETS_EXPORT_NAME}
+        INCLUDES DESTINATION ${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}
+    )
+    install(
+        EXPORT ${NLOHMANN_JSON_TARGETS_EXPORT_NAME}
+        NAMESPACE ${PROJECT_NAME}::
+        DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
+    )
 endif()
-export(
-    TARGETS ${NLOHMANN_JSON_TARGET_NAME}
-    NAMESPACE ${PROJECT_NAME}::
-    FILE ${NLOHMANN_JSON_CMAKE_PROJECT_TARGETS_FILE}
-)
-install(
-    TARGETS ${NLOHMANN_JSON_TARGET_NAME}
-    EXPORT ${NLOHMANN_JSON_TARGETS_EXPORT_NAME}
-    INCLUDES DESTINATION ${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}
-)
-install(
-    EXPORT ${NLOHMANN_JSON_TARGETS_EXPORT_NAME}
-    NAMESPACE ${PROJECT_NAME}::
-    DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
-)