brltty sets -std=gnu99 which the Solaris headers view as incompatible with
_XOPEN_SOURCE < 600, since XPG5 and prior standards required C89, not C99.

This could be avoided if we'd relax system header checks (bug 23506995).

Accepted upstream for release after 5.4:
https://github.com/brltty/brltty/pull/84

diff --git a/configure.ac b/configure.ac
index c28a9da..97c9828 100644
--- a/configure.ac
+++ b/configure.ac
@@ -287,6 +287,9 @@ in
    linux*|gnu*|kfreebsd*)
       brltty_prog_cc_sysflags="-D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=2 -D_BSD_SOURCE -D_XOPEN_SOURCE=500 -D_XOPEN_SOURCE_EXTENDED -D_GNU_SOURCE"
       ;;
+   solaris2.1?) # Solaris 10 & 11 require _XOPEN_SOURCE=600 if using C99
+      brltty_prog_cc_sysflags="-D_XOPEN_SOURCE=600 -D__EXTENSIONS__"
+      ;;
    solaris*)
       brltty_prog_cc_sysflags="-D_XOPEN_SOURCE=500 -D__EXTENSIONS__"
       ;;
diff --git a/Drivers/Braille/EuroBraille/eutp_brl.c b/Drivers/Braille/EuroBraille/eutp_brl.c
index c9f5b29..69ea68b 100644
--- a/Drivers/Braille/EuroBraille/eutp_brl.c
+++ b/Drivers/Braille/EuroBraille/eutp_brl.c
@@ -5,7 +5,9 @@
 ** Login   <[email protected]>
 */
 
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 500
+#endif
 
 /* globals */
 unsigned char extensions[] = {'K', 'L', 'B', 'T', 'A'};
diff --git a/Drivers/Braille/EuroBraille/eutp_transfer.c b/Drivers/Braille/EuroBraille/eutp_transfer.c
index c5c0745..266c4f9 100644
--- a/Drivers/Braille/EuroBraille/eutp_transfer.c
+++ b/Drivers/Braille/EuroBraille/eutp_transfer.c
@@ -7,7 +7,9 @@
 ** Started on  Sun Mar 20 16:10:06 2005 Olivier BERT
 Last update Fri Jun  1 15:23:17 2007 Olivier BERT
 */
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 500
+#endif
 #include <sys/types.h>
 #include <dirent.h>
 #include <string.h>