Build with -D_XOPEN_SOURCE for a declaration of strptime in <time.h>.
This avoids build failures with future compilers which do not support
implicit function declarations.

We cannout build with -D_GNU_SOURCE as the sources contain a defintion
of basename which does not match the GNU prototype.

Submitted upstream: <https://sourceforge.net/p/wput/patches/24/>

diff --git a/src/Makefile.in b/src/Makefile.in
index e7c4c431f50984ec..348807fb9df5bb86 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -5,7 +5,8 @@ prefix=@prefix@
 bindir=@bindir@
 localedir=$(prefix)/share/locale
 CC=@CC@
-CFLAGS= @CFLAGS@ -DLOCALEDIR=\"$(localedir)\" @GNUTLS_CFLAGS@
+CFLAGS= @CFLAGS@ -D_DEFAULT_SOURCE -D_XOPEN_SOURCE \
+  -DLOCALEDIR=\"$(localedir)\" @GNUTLS_CFLAGS@
 LIBS=@LIBINTL@ @LIBS@ @GNUTLS_LIBS@
 EXE=../wput
 GETOPT=@GETOPT@