Refactor cmake checks for DBUS_VA_COPY and DBUS_VA_COPY_ARRAY
For test case execution, CheckCSourceCompiles is now used instead of try_compile and the determination of DBUS_VA_AS_ARRAY is performed with a separate test instead of evaluating the result of HAVE_VA_COPY and HAVE___VA_COPY.
The tests are performed for all supported compilers. Since older MSVC compilers (< 2013) do not support va_copy(), the macro _DBUS_VA_ASSIGN(a1,a2) with the implementation { a1 = a2; } is used as a fallback.
Bug: #117 follow up from !18 (merged)