forced X/Open mode on HPUX removed. The correct way (that affects not only C but also C++ and linking) would be

CFLAGS="-D_XOPEN_SOURCE -D_XOPEN_SOURCE_EXTENDED -D_HPUX_SOURCE"
CXXFLAGS="-D_XOPEN_SOURCE -D_XOPEN_SOURCE_EXTENDED -D_HPUX_SOURCE"
LDFLAGS="-lxnet"

but apparently MySQL works without too.