Remove D_XOPEN_SOURCE=700 which is causing warnings for every src/sbbs3/*

about redefining _XOPEN_SOURCE on every command-line (because pkt-config
is also defining it, to 600).

src/build/Common.gmake

@@ -328,7 +328,7 @@ endif
 
 # PThread-specific flags
 ifeq ($(os),linux)    # Linux
- CFLAGS	+=	-DPOSIX_C_SOURCE=200809L -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700 -DSPEED_MACROS_ONLY -D_GNU_SOURCE
+ CFLAGS	+=	-DPOSIX_C_SOURCE=200809L -D_DEFAULT_SOURCE -D_BSD_SOURCE -DSPEED_MACROS_ONLY -D_GNU_SOURCE
  ifndef THREADS_ACTUALLY_WORK
   CFLAGS    += -D_THREAD_SUID_BROKEN
  endif