ext-toolchain-wrapper.c: Handle an arbitrary amount of arguments

Even though MAXARGS 1000 seems large, it wasn't enough for at least
QtWebKit package. This new version does not have any predefined limits.

Closes #3907

Many thanks to Thomas for tracing the source of the build error.

[Peter: Return rather than abort()]
Signed-off-by: Daniel Nyström <daniel.nystrom@timeterminal.se>
Reported-by: Thomas Björk <thomas.bjork@home.se>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>