[Backport release-2.18] Use FetchContent to get vcpkg if we are not i…

…n a Git repository. (#4529)

Backport 1f4ed0a from #4484.

---
TYPE: BUILD
DESC: Fix errors when building from a directory that is not a Git
repository.

cmake/Options/TileDBToolchain.cmake

@@ -15,11 +15,27 @@ if(DEFINED ENV{VCPKG_ROOT})
     set(CMAKE_TOOLCHAIN_FILE
         "$ENV{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake"
         CACHE STRING "Vcpkg toolchain file")
-else()
+# Try to initialize the submodule only if we are in a Git repository.
+elseif(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/../../.git")
     include(init-submodule)
     set(CMAKE_TOOLCHAIN_FILE
         "${CMAKE_CURRENT_SOURCE_DIR}/external/vcpkg/scripts/buildsystems/vcpkg.cmake"
         CACHE STRING "Vcpkg toolchain file")
+elseif(NOT DEFINED ENV{TILEDB_DISABLE_AUTO_VCPKG})
+    # Inspired from https://github.com/Azure/azure-sdk-for-cpp/blob/azure-core_1.10.3/cmake-modules/AzureVcpkg.cmake
+    message("TILEDB_DISABLE_AUTO_VCPKG is not defined. Fetch a local copy of vcpkg.")
+    # To help with resolving conflicts, when you update the commit, also update its date.
+    # Also make sure the externals/vcpkg submodule is updated to the same commit.
+    set(VCPKG_COMMIT_STRING 1b4d69f3028d74401a001aa316986a670ca6289a) # 2023-09-27
+    message("Vcpkg commit string used: ${VCPKG_COMMIT_STRING}")
+    include(FetchContent)
+    FetchContent_Declare(
+        vcpkg
+        GIT_REPOSITORY https://github.com/microsoft/vcpkg.git
+        GIT_TAG        ${VCPKG_COMMIT_STRING}
+        )
+    FetchContent_MakeAvailable(vcpkg)
+    set(CMAKE_TOOLCHAIN_FILE "${vcpkg_SOURCE_DIR}/scripts/buildsystems/vcpkg.cmake" CACHE STRING "Vcpkg toolchain file")
 endif()
 
 if(APPLE AND NOT DEFINED VCPKG_TARGET_TRIPLET)