GNULIB_POSIXCHECK, remove, and gl_oset

[Eric Blake]

Bruno,

I tried compiling m4 with -DGNULIB_POSIXCHECK, to see if I was missing any 
useful modules.  To my surprise, gl_oset.h failed to compile:

In file included from ../../lib/gl_avltree_oset.h:21,
                 from ../../src/output.c:27:
../../lib/gl_oset.h:236:76: macro "remove" passed 2 arguments, but takes just 1

POSIX states that:
Any function declared in a header may also be implemented as a macro defined in 
the header, so a function should not be declared explicitly if its header is 
included. Any macro definition of a function can be suppressed locally by 
enclosing the name of the function in parentheses, because the name is then not 
followed by the <left-parenthesis> that indicates expansion of a macro function 
name.

In other words, it is legal for a system to #define remove(x), and so any use 
of the name remove as a user identifier must be tolerant of the existence of 
such a function-like macro.

I see two options - either rewrite gl_oset.h to call vtable->(remove)(set,elt), 
or rewrite gl_oset.h and clients to use a different vtable member function 
pointer name that does not clash with the standardized name remove.  Which 
would you prefer?

Meanwhile, the workaround for m4 is to import the remove module, so that it 
does not pick up the GNULIB_POSIXCHECK function-like definition of remove 
(although this is just bulk if m4 itself doesn't use remove).

-- 
Eric Blake