libunibreak: add missing __has_attribute definitions for old compilers

__has_attribute has been introduced with gcc 5 [1].

[1] https://www.gnu.org/software/gcc/gcc-5/changes.html

Fixes:
http://autobuild.buildroot.net/results/64ab825048fac1654b0d0698740ddf857fcc0afa/build-end.log
Signed-off-by: Cedric BAIL <cedric.bail@free.fr>

src/static_libs/libunibreak/wordbreak.c

@@ -209,6 +209,9 @@ static void set_wordbreaks(
                 posLast = posCur;
                 break;
             }
+#ifndef __has_attribute
+# define __has_attribute(x) 0
+#endif
 #if __has_attribute(fallthrough)
            __attribute__((fallthrough));
 #endif
@@ -325,6 +328,9 @@ static void set_wordbreaks(
                 wbcSeqStart = wbcCur;
                 posLast = posCur;
             }
+#ifndef __has_attribute
+# define __has_attribute(x) 0
+#endif
 #if __has_attribute(fallthrough)
            __attribute__((fallthrough));
 #endif