c_attach & const fix

Hi Al.

Guilherme found a problem in c_attach.cc. Here's his patch [1].

apparently, the some llvm has trouble with

  std::map<std::string, void*>::iterator x=attach_list.begin();

after c_attach.cc line 30:

  std::map<const std::string, void*> attach_list;
           ^^^^^

i don't fully understand "const" here. does it have any effect? either
way, i'd like to fix this in unstable ... :)

thanks
felix

[1] http://git.savannah.gnu.org/cgit/gnucap.git/commit/?h=cmake-1&id=8a280668863f5d2ce5aa7508c90346e514edde8e