if i run: $ CFLAGS='-O2 -DMY_CFLAGS' CPPFLAGS='-DMY_CPPFLAGS' emerge IO-AIO ... x86_64-pc-linux-gnu-gcc -c -D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -O2 -march=amdfam10 -pipe -g -frecord-gcc-switches -Wimplicit-function-declaration -DVERSION=\"4.32\" -DXS_VERSION=\"4.32\" -fPIC "-I/usr/lib64/perl5/5.22.0/x86_64-linux-thread-multi/CORE" AIO.c ... it does not use these CFLAGS/CPPFLAGS. instead, it uses the settings that perl itself was apparently compiled with.
*** This bug has been marked as a duplicate of bug 261375 ***