[BUG] Duplicate templates using MSYS2 GNU C++ compiler

[corradomio]

It is possible to compile the library using MSYS2 GNU C++.
However there is a problem on how GNU C++ and Microsoft CL handle the template instantiation.

The problem is located in WSTP .h/.c. files, in the template definitions as (for example in Get.h)

template<>
	GetArray<unsigned char>::Func GetArray<unsigned char>::ArrayF = WSGetInteger8Array;

MSYS2 GNU C++ define the macro "_WIN32". This means that in "Get.c/Get.h" is active the "else" part of

#ifndef _WIN32
...
#else

	template<>
	GetArray<unsigned char>::Func GetArray<unsigned char>::ArrayF = WSGetInteger8Array;
        ...
#endif

When GNU C++ compiler includes the .h in ANOTHER project, it instantiate the template.
BUT the template is ALREADY instantiated and present in "libLLU.a", the library created compiling these source files.
This generates a linker error for "multiple instances of the same template".

The solution can be simple: it is enough to replace

#ifndef _WIN32

with

#if !defined(_WIN32) || defined(__GNUC__)

where GNUC is a unique predefined macro in GNU C/C++ compiler, in this way, the templates are instantiates inside *.c and made them available as "extern template" in the other projects.