[LyX/master] Fix building on cygwin after 23dbcb59

[Enrico Forestieri]

commit f70dd65bb2974fda462b565695c18cef7769d4e0
Author: Enrico Forestieri <for...@lyx.org>
Date:   Thu Jul 7 23:37:14 2016 +0200

    Fix building on cygwin after 23dbcb59
---
 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/configure.ac b/configure.ac
index d9be47d..75df6c7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -317,6 +317,7 @@ char * strerror(int n);
 #define BOOST_NO_WSTRING 1
 
 #ifdef __CYGWIN__
+#  define _DEFAULT_SOURCE
 #  define NOMINMAX
 #  define BOOST_POSIX 1
 #  define BOOST_POSIX_API 1