Fix wrong header included in .hpp files

They should not include the corresponding "-decl" header directly,
because there are not include guards in generated headers.
Generate different code for C++

For C, nothing is changed. In case of C++:

o constants are defined using the "const" keyword instead of evil macros
o native function declarations don't use macros but rather define the functions
  through their exported counterparts (those beginning with "sampgdk_")

This allows use to put the declarations code into a namespace which in turn
will resolve conflicts with existing Win32 API functions (though I don't think
that was a big problem).

Also see issue #37.
Put C++ code into the "sampgdk" namespace

I believe this change is OK since it doesn't break the binary compatibility
although the source compatibility will not retain. Perhaps I should add
a macro that would enable this feature so the old code will still compile.
Add ability to switch namespaces on/off with SAMPGDK_USE_NAMESPACE