examples: declare source as GNU for strdup definition
examples: declare source as GNU for strdup definition

otherwise to show the following warning if compiled in -ansi mode :

  simple.c:34: warning: implicit declaration of function ‘strdup’
  simple.c:34: warning: assignment makes pointer from integer without a cast

Signed-off-by: Carlo Marcelo Arenas Belon <a799130fe4873cad0b28326d5f5ccb2cdc6e1724@sajinet.com.pe>

peda-r/libconfuse,peda-r/libconfuse,troglobit/libconfuse,troglobit/libconfuse,martinh/libconfuse,westermo/libconfuse,westermo/libconfuse,martinh/libconfuse