...
DEBUG: configure:10777: /usr/bin/x86_64-pc-linux-gnu-gcc -std=gnu99 -c -march=native -pipe -fno-strict-aliasing -fno-math-errno -pthread conftest.c 1>&5
DEBUG: configure: In function 'main':
DEBUG: configure:10769:23: error: #error "Insufficient libpng version (10629 required)."
DEBUG: configure: failed program was:
DEBUG: #line 10763 "configure"
DEBUG: #include "confdefs.h"
DEBUG: #include <stdio.h>
DEBUG: #include <sys/types.h>
DEBUG: #include <png.h>
DEBUG: int main() {
DEBUG: #if PNG_LIBPNG_VER < 10629
DEBUG: #error "Insufficient libpng version (10629 required)."
DEBUG: #endif
DEBUG: #ifndef PNG_UINT_31_MAX
DEBUG: #error "Insufficient libpng version."
DEBUG: #endif
DEBUG: ; return 0; }
DEBUG: configure: error: --with-system-png requested but no working libpng found
ERROR: old-configure failed
*** Fix above errors and then restart with\
"make -f client.mk build"
client.mk:379: recipe for target 'configure' failed
make: *** [configure] Error 1

# equery l -po libpng
* Searching for libpng ...
[--O] [ ] media-libs/libpng-1.2.57:1.2
[--O] [ ] media-libs/libpng-1.5.28:1.5
[--O] [ ] media-libs/libpng-1.6.27:0/16
[I-O] [ ] media-libs/libpng-1.6.28:0/16