Weak c++ symbols refresher needed

[Mathieu Malaterre <malat@debian.org>]

Hi,

I am starring at bug #758572. Basically OP reports that `cmake` is
underlinked, which is a serious issue as per policy. However when
reading the details it appears that this is a c++ weak symbol (AFAIK
no weak default definition is available). This weak symbol is
generated by default by gcc when using part of the STL (See
#758572#13).

So I would be tempted to simply close the bug as invalid, since weak
symbol without default definition should not be an issue. However OP
reports that it makes `cmake` fails using a custom `libcurl`.

Could someone please remind me in which case weak symbols (no weak
default definition) can trigger an undefined behavior at runtime ?

Thanks.