Another attempt at fixing the plugin build.

STARPU_GCC_PLUGIN_SUPPORT is used within an "if" in configure.ac.

configure.ac

@@ -1331,6 +1331,8 @@ if test "x$enable_gcc_plugin" = "xyes" -o "x$enable_gcc_plugin" = "xmaybe" ; the
 	AC_SUBST([GCC_PLUGIN_PKGCONFIG])
     fi
 fi
+AM_CONDITIONAL([HAVE_PTR_DEREFS_MAY_ALIAS_P],
+               [test "x$ac_cv_have_decl_ptr_derefs_may_alias_p" = "xyes"])
 
 
 if test "x$ac_cv_have_gcc_plugins" = "xyes" ; then

m4/gcc.m4

@@ -207,8 +207,6 @@ AC_DEFUN([STARPU_GCC_PLUGIN_SUPPORT], [
     AC_SUBST([GCC_FOR_PLUGIN_LIBTOOL_TAG])
   fi
 
-  AM_CONDITIONAL([HAVE_PTR_DEREFS_MAY_ALIAS_P],
-                 [test "x$ac_cv_have_decl_ptr_derefs_may_alias_p" = "xyes"])
 
   AC_SUBST([GCC_PLUGIN_INCLUDE_DIR])
 ])