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 ************************************************/

sqlite3-binding.h

@@ -30,6 +30,7 @@
 ** the version number) and changes its name to "sqlite3.h" as
 ** part of the build process.
 */
+#ifndef USE_LIBSQLITE3
 #ifndef SQLITE3_H
 #define SQLITE3_H
 #include <stdarg.h>     /* Needed for the definition of va_list */
@@ -10338,5 +10339,9 @@ struct fts5_api {
 #endif
 
 #endif /* _FTS5_H */
+#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.h *********/

sqlite3ext.h

@@ -1,3 +1,4 @@
+#ifndef USE_LIBSQLITE3
 /*
 ** 2006 June 7
 **
@@ -12,7 +13,7 @@
 ** This header file defines the SQLite interface for use by
 ** shared libraries that want to be imported as extensions into
 ** an SQLite instance.  Shared libraries that intend to be loaded
-** as extensions by SQLite should #include this file instead of 
+** as extensions by SQLite should #include this file instead of
 ** sqlite3.h.
 */
 #ifndef SQLITE3EXT_H
@@ -543,14 +544,14 @@ typedef int (*sqlite3_loadext_entry)(
 #endif /* !defined(SQLITE_CORE) && !defined(SQLITE_OMIT_LOAD_EXTENSION) */
 
 #if !defined(SQLITE_CORE) && !defined(SQLITE_OMIT_LOAD_EXTENSION)
-  /* This case when the file really is being compiled as a loadable 
+  /* This case when the file really is being compiled as a loadable
   ** extension */
 # define SQLITE_EXTENSION_INIT1     const sqlite3_api_routines *sqlite3_api=0;
 # define SQLITE_EXTENSION_INIT2(v)  sqlite3_api=v;
 # define SQLITE_EXTENSION_INIT3     \
     extern const sqlite3_api_routines *sqlite3_api;
 #else
-  /* This case when the file is being statically linked into the 
+  /* This case when the file is being statically linked into the
   ** application */
 # define SQLITE_EXTENSION_INIT1     /*no-op*/
 # define SQLITE_EXTENSION_INIT2(v)  (void)v; /* unused parameter */
@@ -558,3 +559,7 @@ typedef int (*sqlite3_loadext_entry)(
 #endif
 
 #endif /* SQLITE3EXT_H */
+#else // USE_LIBSQLITE3
+ // If users really want to link against the system sqlite3 we
+// need to make this file a noop.
+ #endif