feat: switch datetime package to an external dependency (#83)

By switching to an external dependency controlled by a central file we
can more cleanly reference date::date and date::tz. We could do the same
by "externally loading" a submodule but either way we end up downloading
the datetime repo.

src/substrait/textplan/converter/CMakeLists.txt

@@ -1,5 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
+include(../../../../third_party/datetime.cmake)
+
 set(TEXTPLAN_SRCS
     InitialPlanProtoVisitor.cpp
     InitialPlanProtoVisitor.h

src/substrait/textplan/parser/CMakeLists.txt

@@ -1,5 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
+include(../../../../third_party/datetime.cmake)
+
 add_subdirectory(grammar)
 
 add_library(

third_party/CMakeLists.txt

@@ -5,8 +5,7 @@ if(NOT ${ABSL_INCLUDED_WITH_PROTOBUF})
   add_subdirectory(abseil-cpp)
 endif()
 
-set(BUILD_TZ_LIB ON)
-add_subdirectory(datetime)
+include(datetime.cmake)
 
 add_subdirectory(fmt)
 add_subdirectory(googletest)

third_party/datetime.cmake

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include_guard(GLOBAL)
+
+set (BUILD_TZ_LIB ON CACHE BOOL "timezone library is a dependency" FORCE)
+include(FetchContent)
+FetchContent_Declare(date_src
+        GIT_REPOSITORY https://github.com/HowardHinnant/date.git
+        GIT_TAG v3.0.1
+        )
+FetchContent_MakeAvailable(date_src)