Use hard-coded SDK path for system module map

We've been down this path before. Unfortunately,
`/usr/include/sqlite3.h` isn't reliably available on all Macs. Because
module maps don't yet have dynamic support (something that the Swift
Package Manager intends to have solved soon), we need to hardcode the
SDK path (or use a script to substitute it).

Fixes stephencelis#319.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>

Author: stephencelis

SQLite3/module.modulemap

@@ -1,5 +1,8 @@
 module SQLite3 [system] {
-    header "/usr/include/sqlite3.h"
+    // Requires latest Xcode installed in standard path.
+    header "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/sqlite3.h"
+
     link "sqlite3"
+
     export *
 }