mongoose: bump to version 5.3

Mongoose has changed quite drastically in the last months.
Here is a list of changes which had to be applied:

 - Bump version from 3.7 to 5.3
 - use tarball download via github helper iso git clone
 - need largefile support
 - compilation takes place into "examples"
 - weberver source is now "server.c"
 - adapt to new command line options
 - SSL support now controlled via NS_ENABLE_SSL

[Peter: extend commit text, use CFLAGS_EXTRA, only build server]
Signed-off-by: Davide Viti <zinosat@tiscali.it>
Cc: Will Wagner <will_wagner@carallon.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>