I'm sorry to just post this excerpt of the build log, but I fixed this in my tree and I'm trying to catch my system back up with the portage tree.
checking for working do while(0) macros... yes
checking for ISO C99 varargs macros in C... yes
checking for ISO C99 varargs macros in C++... yes
checking for GNUC varargs macros... yes
checking for GNUC visibility attribute... ./configure: line 30513: syntax error: unexpected end of file
!!! Please attach the following file when seeking support:
* ERROR: dev-libs/glib-2.20.5-r1 failed:
The problem is demonstrated in the ./configure excerpt I will attach.
The problem is fixed by http://git.gnome.org/browse/glib/commit/?id=c24f6e55276fac68b08df34b0d350f65afe994d9 . It looks like this problem may only affect people who are selectively unstable rather than completely unstable (glib-2.22* is probably fixed; I checked that glib-2.22.4's acglib.m4 file has the fix). It would be nice if this fix was backported in case if autoconf-2.65 is stablized before glib, I suppose...
Created attachment 222039 [details]
This ./configure excerpt was post-processed using emac's indentation feature to help me find the problem and to better demonstrate it.
Look for the string ``fiac_compile'' on line 62. That's supposed to be ``fi ac_compile''. There's a newline that's eaten by a ``dnl'' m4 command which is removed in the git commit (linked above).
glib-2.22 is already stable on amd64/x86, I don't see why we should touch 2.20 versions. See bug 304777
Closing fixed, please upgrade.