Assorted fixes for Cygwin:

Eliminate the mysterious games that the Cygwin build plays with the linker
flag variables.  DLLLIBS is gone, use SHLIB_LINK like everyone else.
Detect cygipc in configure, after the linker flags are set up, otherwise
configure might not work at all.

Make sure everything is covered by make clean.

Fix the build of the new conversion procedure modules.

Add new DLLIMPORT markers where required.

Finally, the compiler complains if we use an explicit
-I/usr/local/include, so don't do that.  Curiously, -L/usr/local/lib is
still necessary.