Avoid unconditional definition of _LARGEFILE_SOURCE

On some build systems (i.e. Buildroot), _LARGEFILE_SOURCE is defined
at "higher level" as default CFLAG definition.
This fix avoids the following compilation warning:

mongoose-5.4/mongoose.c:52:0: warning: "_LARGEFILE_SOURCE" redefined [enabled by default]
 #define _LARGEFILE_SOURCE       // Enable fseeko() and ftello() functions
 ^
<command-line>:0:0: note: this is the location of the previous definition