Add USE_SYSTEM_DATE cmake option (#372)

* Add `USE_SYSTEM_DATE` cmake option

CMakeLists.txt

@@ -32,6 +32,14 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/")
 
 include(CTest)
 
+option(USE_SYSTEM_DATE "\
+    Use find_package to find installed HowardHinnant's \
+    date library instead of fetching it from github" OFF
+)
+
+if(USE_SYSTEM_DATE)
+    find_package(date REQUIRED)
+endif()
 
 ### Dependencies 
 add_subdirectory(dependencies)

README.md

@@ -135,7 +135,7 @@ To demonstrate that sqlpp11 can work with other backends as well, here is an exp
   * STL Container: https://github.com/rbock/sqlpp11-connector-stl
 
 __Date Library:__
-sqlpp11 requires [Howard Hinnant's date library](https://github.com/HowardHinnant/date) for `date` and `date_time` data types. Sqlpp11 uses FetchContent to pull the library automatically in the project.
+sqlpp11 requires [Howard Hinnant's date library](https://github.com/HowardHinnant/date) for `date` and `date_time` data types. By default, sqlpp11 uses FetchContent to pull the library automatically in the project. If you want to use an already installed version of the library with `find_package`, set `USE_SYSTEM_DATE` option to `ON`.
 
 Build and Install
 -----------------

dependencies/CMakeLists.txt

@@ -22,11 +22,14 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-include(FetchContent)
 
-FetchContent_Declare(date
-    GIT_REPOSITORY https://github.com/HowardHinnant/date.git
-    GIT_TAG        v3.0.0
-) 
+if(NOT USE_SYSTEM_DATE)
+    include(FetchContent)
 
-add_subdirectory(hinnant_date)
+    FetchContent_Declare(date
+        GIT_REPOSITORY https://github.com/HowardHinnant/date.git
+        GIT_TAG        v3.0.0
+    )
+
+    add_subdirectory(hinnant_date)
+endif()