Warning about direct access to weak symbols

[Jacob Carlborg]

When compiling the runtime for iOS using `ldc-build-runtime` I get a lot 
of warnings looking like this:

ld: warning: direct access in function 'ltmp2' from file 
'objects-unittest-debug/std/algorithm/comparison.o' to global weak 
symbol 
'__D3std3uni__T23switchUniformLowerBoundSQBl10functional__T9binaryFunVAyaa6_61203c3d2062VQta1_61VQBba1_62ZQBvTAxkTkZQDxFNaNbNiNfQskZm' 
from file 'objects-unittest-debug/std/algorithm/comparison.o' means the 
weak symbol cannot be overridden at runtime. This was likely caused by 
different translation units being compiled with different visibility 
settings.

-- 
/Jacob Carlborg