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

[brauner]

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 [email protected]

Closes #330.

After the patch:

1. linking against system sqlite3:

chb@conventiont|~/source/go/src/github.com/mattn/go-sqlite3|2016-10-17/fix_libsqlite3_build>
> go build -tags libsqlite3 _example/simple/simple.go
chb@conventiont|~/source/go/src/github.com/mattn/go-sqlite3|2016-10-17/fix_libsqlite3_build %>
> ldd simple | grep sqlite
        libsqlite3.so.0 => /usr/lib/x86_64-linux-gnu/libsqlite3.so.0 (0x00007f860d0a4000)

1. using vendored sqlite3:

chb@conventiont|~/source/go/src/github.com/mattn/go-sqlite3|2016-10-17/fix_libsqlite3_build %>
> go build _example/simple/simple.go
chb@conventiont|~/source/go/src/github.com/mattn/go-sqlite3|2016-10-17/fix_libsqlite3_build %>
> ldd simple | grep sqlite

[coveralls]

Coverage remained the same at 62.274% when pulling 78124cb on brauner:2016-10-17/fix_libsqlite3_build into e5a3c16 on mattn:master.

[brauner]

@mattn ping. We'd really like to see this issue resolved. Would be great if you could take a look whether this pr is enough. :)

[mattn]

please update tool/upgrade.go too

[brauner]

@mattn, done.

[coveralls]

Coverage decreased (-0.6%) to 61.668% when pulling 0554f97 on brauner:2016-10-17/fix_libsqlite3_build into e5a3c16 on mattn:master.

[coveralls]

Coverage decreased (-0.6%) to 61.668% when pulling 0554f97 on brauner:2016-10-17/fix_libsqlite3_build into e5a3c16 on mattn:master.

[coveralls]

Coverage decreased (-0.9%) to 61.668% when pulling 420dc66 on brauner:2016-10-17/fix_libsqlite3_build into 955dae4 on mattn:master.

[mattn]

LGTM!

[mattn]

Thanks!

[marienz]

See also #318. This was previously handled using a +build !libsqlite3 build tag. The go tool recognizes such tags even in non-Go source, and does not invoke the C compiler at all. I'd argue that's a tidier solution than this one, which runs the C compiler on an effectively empty file.