Allow direct cast of get_proc_address result (#95)

GetProcAddress returns a pointer to some function. It can return
pointers to different functions, so it has to return something that is
suitable to store any pointer to function. Microsoft chose FARPROC,
which is int (WINAPI *)() on 32-bit Windows. The user is supposed to
know the signature of the function he requests and perform a cast
(which is a nop on this platform). The result is a pointer to function
with the required signature, which is bitwise equal to what
GetProcAddress returned. However, gcc >= 8 warns about that.

Add an overload with a template parameter to do the cast inside
the function and suppress the warning there.

include/boost/winapi/get_proc_address.hpp

@@ -72,6 +72,19 @@ BOOST_FORCEINLINE FARPROC_ get_proc_address(HMODULE_ hModule, LPCSTR_ lpProcName
 #endif
 }
 
+template<typename Signature>
+BOOST_FORCEINLINE Signature get_proc_address(HMODULE_ hModule, LPCSTR_ lpProcName)
+{
+#if defined(BOOST_GCC) && BOOST_GCC >= 80000
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wcast-function-type"
+#endif
+    return reinterpret_cast<Signature>(get_proc_address(hModule, lpProcName));
+#if defined(BOOST_GCC) && BOOST_GCC >= 80000
+#pragma GCC diagnostic pop
+#endif
+}
+
 } // namespace winapi
 } // namespace boost