clean up references to `__IMPORT__` and `__EXPORT__`

clean up references to `__IMPORT__` and `__EXPORT__`

This removes any reference to the custom `__IMPORT__` and `__EXPORT__`
macros.  These are remnants from supporting uwin.  However, the need for
uwin is greatly diminished by proper native Windows support for the
library.

Furthermore, the functionality that this enables will be restored
imminently by means of decoration of public interfaces.

The uwin system used `_BLD_STATIC` to indicate a static build and
`__DYNAMIC__` to indicate a shared build.  Furthermore, it used
`_BLD_DLL` to indicate a dynamic library on Windows, that is a DLL.
This augmented the check of `_DLL` which is the Microsoft specified
define that indicates a shared library build.  Based upon the state of
these four flags (why four flags for 2 states?) it then defined
`__IMPORT__` and `__EXPORT__` to `__declspec(dllimport)` and
`__declspec(dllexport)` respectively.

To further complicate things, UWIN was built up during Windows 3.1,
which meant that it predated the linker support for DLL storage.
Instead, this is required work by the user, and it has special macros
for declaring the (now) synthetic import symbol as well as defining it.
This is now no longer required due to improved support in the compiler
and linker which will perform the necessary work.

On Unix, this synthetic symbol declaration and definition is not
required as the linker has always supported this for shared libraries -
it is responsible for creating the PLT which is roughly equivalent to
the IAT (technically, the IAT is both the PLT and the GOT).  This is now
also automated on Windows, at least for the past ~25 years.

This clean up is meant to simplify the migration to `[module]_EXPORTS`
as the "standard" macro which is defined by CMake (and is generally used
elsewhere now as well) to indicate that the symbols for the module
should be exported.  `_DLL` provides the necessary information whether a
DLL is being built or not.

With that history, it is clear that this is a regression for GraphViz on
uwin, the temporary breakage seems reasonable to simplify the migration.