cgosqlite: use POSIX 2014 explicitly

Unless this is defined a strict compiler will error with an implicit
definition error at least, or at worst will not even find a definition
as one may optionally not be provided at all:

```
  ./cgosqlite.h: In function 'monotonic_clock_gettime':
  ./cgosqlite.h:52:9: error: implicit declaration of function 'clock_gettime' [-Wimplicit-function-declaration]
    52 |         clock_gettime(CLOCK_MONOTONIC, t);
        |         ^~~~~~~~~~~~~
```

This resolves an earlier TODO, where crawshaw had observed
CLOCK_MONOTONIC not being defined.

As a very minor change: use the system import syntax for system imports.

cgosqlite/cgosqlite.go

@@ -29,22 +29,17 @@ package cgosqlite
 // #cgo CFLAGS: -DSQLITE_ENABLE_STAT4
 // #cgo CFLAGS: -DSQLITE_ENABLE_DBSTAT_VTAB=1
 // #cgo CFLAGS: -DHAVE_USLEEP=1
-// #cgo linux,!android LDFLAGS: -ldl -lm -lrt
+//
+// // Select POSIX 2014 at least for clock_gettime.
+// #cgo CFLAGS: -D_XOPEN_SOURCE=600
 // #cgo linux CFLAGS: -std=c99
 //
 // // On Android, unlike Linux, there are no separate libpthread or librt
 // // libraries. That functionality is included directly in libc, which does not
 // // need to be explicitly linked against. See
 // // https://developer.android.com/ndk/guides/stable_apis#c_library.
 // #cgo android LDFLAGS: -ldl -lm
-//
-// // TODO(crawshaw): for some reason, I cannot get CLOCK_MONOTONIC
-// // defined properly here by cgo. My C compiler seems otherwise
-// // fine, I can compile a small C program on linux referring to
-// // CLOCK_MONOTONIC.
-// //
-// // For now, use the value directly. It's a fixed value on linux.
-// #cgo linux CFLAGS: -DCLOCK_MONOTONIC=1
+// #cgo linux,!android LDFLAGS: -ldl -lm -lrt
 //
 // #include <stdint.h>
 // #include <stdlib.h>

cgosqlite/cgosqlite.h

@@ -1,5 +1,5 @@
-#include "stdint.h"
-#include "time.h"
+#include <stdint.h>
+#include <time.h>
 
 // uintptr versions of sqlite3 pointer types, to avoid allocations
 // in cgo code. (go/corp/9919)