Am 23. September 2017 20:37:09 MESZ schrieb Stefano Crocco <posta@stefanocrocco.it>:

On Saturday, 23 September 2017 17:53:25 CEST Christoph Böhmwalder wrote:
 Hey all,
 
 I've been trying to get Thunderbird to build for a few days now.
 Since I'm pretty much out of ideas at this point, I figured I'd ask on here
 if someone has an idea on what my problem is.
 
 I've attached the output of `emerge --info
 '=mail-client/thunderbird-52.3.0::gentoo'`, `emerge -pqv
 '=mail-client/thunderbird-52.3.0::gentoo'`, and the complete build log.
 
 From the error message I can see that it likely has something to do with
 either libpng or zlib. I have both of those installed:
 
 $ emerge --info libpng
 --- >8 ---
 media-libs/libpng-1.6.29::gentoo was built with the following:
 USE="apng (-neon) -static-libs" ABI_X86="32 (64) (-x32)" CPU_FLAGS_X86="sse"
 
 $ emerge --info zlib
 --- >8 ---
 sys-libs/zlib-1.2.11::gentoo was built with the following:
 USE="minizip -static-libs" ABI_X86="32 (64) (-x32)"
 
 
 I noticed that I have zlib-1.2.11 installed, though Thunderbird (or libpng?)
 is apparently trying to reference a symbol from zlib 1.2.9. I tried
 downgrading to zlib 1.2.9, but...
 
 # emerge --ask \=sys-libs/zlib-1.2.9
 
 These are the packages that would be merged, in order:
 
 Calculating dependencies... done!
 
 emerge: there are no ebuilds to satisfy "=sys-libs/zlib-1.2.9".
 
 
 I'd really appreciate any hints on what I'm doing wrong here. Thanks!
 
 
 --
 
 Regards,
 Christoph

I believe you should re-emerge libpng because it seems to have been built 
against an old (and incompatible) version of zlib (1.2.9).

I hope this helps

Stefano