https://bugs.gentoo.org/869419

POSIX_C_SOURCE is needed for ftello.
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -266,6 +266,7 @@ add_library(aom_rtcd OBJECT ${AOM_RTCD_SOURCES})
 add_dependencies(aom_rtcd aom_version)
 
 if(ENABLE_EXAMPLES)
+  add_definitions(-D_POSIX_C_SOURCE=200112L)
   add_library(aom_encoder_stats OBJECT ${AOM_ENCODER_STATS_SOURCES})
   set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} aom_encoder_stats)
 endif()