Do not define __EXTENSIONS__ if already defined. [skip ci]

Same as in tcpdump.

./ftmacros.h:48:11: warning: '__EXTENSIONS__' macro redefined
  [-Wmacro-redefined]
<built-in>:347:9: note: previous definition is here

1 files changed, 6 insertions, 1 deletions

diff --git a/ftmacros.h b/ftmacros.h
index 3cd75056..7975463b 100644
--- a/ftmacros.h
+++ b/ftmacros.h
@@ -45,7 +45,12 @@
  * namespace to the maximum extent possible"?
  */
 #if defined(sun) || defined(__sun)
-  #define __EXTENSIONS__
+  /*
+   * On Solaris Clang defines __EXTENSIONS__ automatically.
+   */
+  #ifndef __EXTENSIONS__
+    #define __EXTENSIONS__
+  #endif
 
   /*
    * We also need to define _XPG4_2 in order to get