add build constraint for solaris

fixes #514

sqlite3_solaris.go

@@ -0,0 +1,12 @@
+// Copyright (C) 2018 Yasuhiro Matsumoto <mattn.jp@gmail.com>.
+//
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+// +build solaris
+
+package sqlite3
+
+/*
+#cgo CFLAGS: -D__EXTENSIONS__=1
+*/
+import "C"