autotools: fix dllmain.c in unity builds

Sync it with cmake to:
- exclude it from all builds except Windows and Cygwin.
- exclude it from unity builds for Cygwin to avoid the included
  `windows.h` header interfere with the rest of the code.

Also:
- fix to trim ending spaces from `CSOURCES` for the `tidy` target.
  The solution requires a non-POSIX `-E` `sed` option. Supported by BSD
  and GNU implementations.
  Follow-up to 37523c91bc #16480

Follow-up to 60c3d04465 #14815
Follow-up to 7860f575fe #12408

Closes #16712

lib/Makefile.inc

@@ -146,7 +146,6 @@ LIB_CFILES =         \
   cw-out.c           \
   cw-pause.c         \
   dict.c             \
-  dllmain.c          \
   doh.c              \
   dynbuf.c           \
   dynhds.c           \