[cygwin] Refactor C++ exception handling for Win32 correctness

[cygwin] Refactor C++ exception handling for Win32 correctness

* tests/exception.at (common.h): New file, refactored from
(module.h): Move declaration of modexc to libcommon.
Include common.h. Use explicit import/export markings for
symbols when building on win32 or cygwin.
(common.cpp): New file.
(module.cpp): Ensure correct symbols markings when including
module.h.
(lib.h): Use explicit import/export markings for symbols
when building on win32 or cygwin.
(lib.cpp): Ensure correct symbol markings when including lib.h.
(main.cpp): Include common.h.
(commands): Ensure correct symbol markings when compiling
main.cpp, and lib.cpp, and module.cpp. Add command to compile
common.cpp, and to link libcommon.la.  Add libcommon.la when
linking module.la and main.  Add command to install libcommon.la.

Signed-off-by: Charles Wilson <libtool@cwilson.fastmail.fm>