dtwm: fixes to make it build

dtwm: fixes to make it build

Removed PORT_NOFORK define in favor of autotools vfork availablility
checking. Use vfork if present.

Got rid of getenv/putenv definitions in favor of including correct
system headers.

Use FD_CLOEXEC in fcntl() call rather than hardcoded '1'.

Added -lm check to configure so dtwm will build.

Renamed internal round() function to wmround() to fix warning about
redefinition of builtin.

Fixed missing backslash in Makefile.am that prevented half the sources
from building.