Use reentrant localtime if available

I've seen some segfaults on android app start up with traces like

```
I/DEBUG   ( 2446): backtrace:
I/DEBUG   ( 2446):     #00  pc 00021c88  /system/lib/libc.so (__findenv+71)
I/DEBUG   ( 2446):     #01  pc 00021cc9  /system/lib/libc.so (getenv+4)
I/DEBUG   ( 2446):     #02  pc 000277d7  /system/lib/libc.so
I/DEBUG   ( 2446):     #03  pc 00027e49  /system/lib/libc.so (localtime_r+10)
I/DEBUG   ( 2446):     #04  pc 00be9c44  /data/app-lib/com.pivos.tofu-1/libtofu.so (GetLocalTime(_SYSTEMTIME*)+36)
I/DEBUG   ( 2446):     #05  pc 003d9fb8  /data/app-lib/com.pivos.tofu-1/libtofu.so (CLog::Log(int, char const*, ...)+116)
```

I'm not *sure* that this fixes it, but this fixes a rather blatant thread-safety issue.