rb4: linker resolution patch

[Richard Boulton]

I've found a problem with the determination of the linker to use: if we
have:

foo_SOURCES = foo.c
dist_foo_SOURCES = bar.cxx

then we should use CXXLINK for linking, but currently we will use LINK.

I've put together a test case to check this, and also a patch to fix this.
The fix for the patch is also an architectural change which is needed for
my (rather large) coming patch to remove O(2^n) behaviour when defining
_OBJECTS variables.

All tests behave correctly after this patch is applied.

-- 
Richard

patch1
Description: Text document