actually link to <sqlite3.h> when -tags libsqlite3

Building with -tags libsqlite3 used the sqlite3.h from the system but the go
compiler will compile all *.{c,h} files in the same direcory:

	"When the Go tool sees that one or more Go files use the special import
	"C", it will look for other non-Go files in the directory and compile
	them as part of the Go package. Any .c, .s, or .S files will be compiled
	with the C compiler." (https://golang.org/cmd/cgo/)

So if users actually want to link against the system sqlite3 we should make
sqlite3-binding.* a noop.

Signed-off-by: Christian Brauner <christian.brauner@canonical.com>

sqlite3-binding.c

@@ -17,6 +17,7 @@
 ** language. The code for the "sqlite3" command-line shell is also in a
 ** separate file. This file contains only code for the core SQLite library.
 */
+#ifndef USE_LIBSQLITE3
 #define SQLITE_CORE 1
 #define SQLITE_AMALGAMATION 1
 #ifndef SQLITE_PRIVATE
@@ -197846,5 +197847,9 @@ static int sqlite3Fts5VocabInit(Fts5Global *pGlobal, sqlite3 *db){
 
     
 #endif /* !defined(SQLITE_CORE) || defined(SQLITE_ENABLE_FTS5) */
+#else // USE_LIBSQLITE3
+// If users really want to link against the system sqlite3 we
+// need to make this file a noop.
+#endif
 
 /************** End of fts5.c ************************************************/