Build SQLite3 as part of soci_sqlite3 itself if necessary

Get rid of a separate soci_sqlite3_builtin target as it didn't work
correctly when using static libraries.

Instead, just compile the amalgamated SQLite3 source as part of
soci_sqlite3 itself, which works after the changes of the parent commit
as we don't get tons of warnings for this C file any longer.

src/backends/sqlite3/CMakeLists.txt

@@ -64,18 +64,12 @@ if (NOT TARGET SQLite::SQLite3)
     message(STATUS "Falling back on built-in version of SQLite3")
   endif()
 
-  add_library(soci_sqlite3_builtin)
-  target_sources(soci_sqlite3_builtin
+  target_sources(soci_sqlite3
     PRIVATE
       ${SQLITE3_SOURCE_FILE}
   )
-  target_include_directories(soci_sqlite3_builtin
-    PUBLIC
-      "${SQLITE3_DIRECTORY}
-  )
-
-  target_link_libraries(soci_sqlite3
+  target_include_directories(soci_sqlite3
     PRIVATE
-      soci_sqlite3_builtin
+      ${SQLITE3_DIRECTORY}
   )
 endif()