x86_64-pc-linux-gnu-gcc -I. -I. -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -DPIC -DHAVE_AV_CONFIG_H -march=native -O2 -g0 -march=core2 -std=c99 -fomit-frame-pointer -fPIC -pthread -Wdeclaration-after-statement -Wall -Wno-parentheses -Wno-switch -Wno-format-zero-length -Wdisabled-optimization -Wpointer-arith -Wredundant-decls -Wno-pointer-sign -Wcast-qual -Wwrite-strings -Wtype-limits -Wundef -Wmissing-prototypes -Wstrict-prototypes -O3 -fno-math-errno -fno-signed-zeros -fno-tree-vectorize -Werror=implicit-function-declaration -Werror=missing-prototypes -Werror=declaration-after-statement -MMD -MF libavformat/matroskaenc.d -MT libavformat/matroskaenc.o -c -o libavformat/matroskaenc.o libavformat/matroskaenc.c -O3 overwrites
Did you remember to enable USE=custom-cflags?
my bad, custom-cflags was not enabled