Define the new _DEFAULT_SOURCE macro for glibc >= 2.20

This will also enable SVID stuff, but that appears to be unavoidable.

See the following URIs:
https://lwn.net/Articles/590381/
https://sourceware.org/glibc/wiki/Release/2.20#Deprecation_of__BSD_SOURCE_and__SVID_SOURCE_feature_macros
http://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html#index-_005fDEFAULT_005fSOURCE

src/build/Common.gmake

@@ -298,7 +298,7 @@ endif
 
 # PThread-specific flags
 ifeq ($(os),linux)    # Linux
- CFLAGS	+=	-DPOSIX_C_SOURCE=200809L -D_BSD_SOURCE -D_XOPEN_SOURCE=700 -DSPEED_MACROS_ONLY
+ CFLAGS	+=	-DPOSIX_C_SOURCE=200809L -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700 -DSPEED_MACROS_ONLY
  ifndef THREADS_ACTUALLY_WORK
   CFLAGS    += -D_THREAD_SUID_BROKEN
  endif