Log Message:
haproxy: Rework SunOS XOPEN_SOURCE handling.

This package explicitly avoids C99-isms and fails to build in C99 mode with
older compilers, even though it works fine with newer compilers that default
to C99 mode.  So, we need to explicitly request XPG4_2 if and only if the
compiler does not default to C99, and must not require C99.