GCC allows redefinition of an "extern inline" function in GNU89 mode, accepting the following code:

  extern inline int f(int a) { return a; }
  int f(int b) { return b; }

which Clang rejects with

$ clang -std=gnu89 foo.c
foo.c:3:5: error: redefinition of 'f'
int f(int b) { return b; }
    ^
foo.c:1:19: note: previous definition is here
extern inline int f(int a) { return a; }
                  ^
2 diagnostics generated.

We previously decided that we would not implement this feature (see bug #4719), but apparently this extension is used by Wine:

  http://lists.cs.uiuc.edu/pipermail/cfe-dev/2009-October/006766.html

We should reconsider.