gcc: Fix detection of function declarations.

* m4/gcc.m4 (STARPU_GCC_PLUGIN_SUPPORT): Include <gcc-plugin.h> when
  testing for declarations.

m4/gcc.m4

@@ -53,7 +53,8 @@ AC_DEFUN([STARPU_GCC_PLUGIN_SUPPORT], [
     dnl   build_call_expr_loc_vec   -- likewise
     _STARPU_WITH_GCC_PLUGIN_API([
       AC_CHECK_DECLS([build_call_expr_loc_array, build_call_expr_loc_vec],
-        [], [], [#include <tree.h>])
+        [], [], [#include <gcc-plugin.h>
+	         #include <tree.h>])
     ])
   fi