On Linux the following gives out a "multiple definition" error, 
as expected and desired:

    c/test.c:
    void dotest(void) { printf("C\n"); }

    d/test.d:
    extern(C) void dotest() { writeln("D"); }

On OS X no error is flagged, and the C function is always called, 
irrespective of which order I specify the .o files to link. The cause seems to be OS X DMD (v.2.063.2) outputting the function to the S section:

On Ubuntu:

    $ nm d/test.o | grep dotest; echo "--"; nm c/test.o | grep dotest
    0000000000000000 T dotest
    --
    0000000000000000 T dotest

On OS X:

    $ nm d/test.o | grep dotest; echo "--"; nm c/test.o | grep dotest
    0000000000001490 S _dotest   <-- not in text section, as in Linux
    --
    0000000000000000 T _dotest
    0000000000000060 S _dotest.eh

When using LDC on OS X the linking fails, as expected and desired:

    duplicate symbol _dotest in:
        d/test.o
        c/test.o
    ld: 1 duplicate symbol for architecture x86_64

The LDC .o sections are the same as the C version:

    $ nm d/test.o | grep dotest; echo "--"; nm c/test.o | grep dotest
    0000000000000000 T _dotest
    00000000000000b0 S _dotest.eh
    --
    0000000000000000 T _dotest
    0000000000000060 S _dotest.eh