So perhaps the first place to start is with elfdump. If you run
elfdump -v /lib/libc.so
then you'll get a bunch of lines like so:
index version dependency
 libc.so.1 [ BASE ]
 ILLUMOS_0.13 ILLUMOS_0.12
 ILLUMOS_0.12 ILLUMOS_0.11
 ILLUMOS_0.11 ILLUMOS_0.10
 SUNW_0.8 SUNW_0.7
 SUNW_0.7 SYSVABI_1.3
So, these lines correspond to the various released versions of libc. Every time you add a new function to libc, that means a new version of the library. And each version depends on the one before.
These versions are listed in a mapfile, at usr/src/lib/libc/port/mapfile-vers in the illumos source. So, what you can see here is that ILLUMOS_0.13 (which is the version shipped with Tribblix 0m16) is where eventfd got added. If you want strerror_l (which you do if you're building vlc), then you need ILLUMOS_0.14; if you want pthread_attr_get_np (which you need for QT5) then you need ILLUMOS_0.21. (Unfortunately, Tribblix 0m17 only picks up ILLUMOS_0.19.) Looking back, you can see everything exposed by libc and what version of Solaris or illumos it was added in.
Another trick is to run elfdump against a binary. For example:
elfdump -v /usr/gnu/bin/tar
will tell us which versions of which libraries are required. Part of this is:
Version Needed Section: .SUNW_version
index file version
 libnsl.so.1 SUNW_0.7
 libc.so.1 ILLUMOS_0.8
 ILLUMOS_0.1 [ INFO ]
 SUNW_1.23 [ INFO ]
 SUNW_1.22.6 [ INFO ]
This is quite informative. It tells you that gtar calls the newlocale stuff from ILLUMOS_0.8, and a bunch of older stuff. But the point here is that it is calling illumos additions to libc, so this binary won't work on Solaris 10 (and probably not on Solaris 11 either). If you're building binaries for distribution across distros, you can use this information to confirm that you haven't accidentally pulled in functions that might not be available everywhere.