[PATCH] Fix optional variables in libargp

[Fedin Pavel]

 Hello!

> Okay, well I agree that this sounds like a good solution.  For now you
> have a workaround, and I'll be glad to consider a patch if you submit
> one.

 Please take it. This is my experimental implementation which appears to be
simpler than i suggested.

 Some details: this implementation adds explicit dllexport attribute to
these 4 variables. This allows to look them up using GetProcAddress(). The
dllexport attribute has to be added explicitly because when compiling an
.exe file (without -shared switch) gcc does not mark externals as exportable
by default. When building a .dll, all external symbols are marked as
exportable by default, *UNLESS* we have at least one explicit dllexport
specification, hence i surrounded this with #ifndef DLL_EXPORT, this
definition is introduced by libtool when building a shared library.
 This implementation would not work in the following corner cases:
 a) .exe file uses some foo.dll library, which defines these variables and
uses libargp - symbols are picked up only from .exe file itself.
 b) Building that foo.dll manually without DLL_EXPORT definition can get
broken - the compiler will see dllexport attribute, and it will override the
default behavior to make all externs exportable.
 I believe these cases are quite unusual so it's OK. Getting rid of these
limitations would need more complex implementation with more changes.

 I have tested the implementation on x86-64 with RedHat's Prelink utility,
and it works quite fine. i386 should work too, but please retest, just in
case.

 Additionally, i added the second #ifdef _WIN32 in order to make the same
thing working also for MinGW target.

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia
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