Add macros to handle constructor functions on the compilers that support it

Add macros to handle constructor functions on the compilers that support it

This is only supported on some compilers, so we define G_HAS_CONSTRUCTORS
when it is supported. However, when it is supported we guarantee that
both constructors and destructors work, in executables as well as shared
libraries (including runtime unloading of shared libraries).

Usage is a bit unorthodox, as some compilers need to use #pragma to
implement constructors, and #pragma can't be used in macros.

The canonical way to use this:

  #ifdef G_DEFINE_CONSTRUCTOR_NEEDS_PRAGMA
  #pragma G_DEFINE_CONSTRUCTOR_PRAGMA_ARGS(my_constructor)
  #endif
  G_DEFINE_CONSTRUCTOR(my_constructor)

  static void my_constructor (void)
  {
    ...

  #ifdef G_DEFINE_DESTRUCTOR_NEEDS_PRAGMA
  #pragma G_DEFINE_DESTRUCTOR_PRAGMA_ARGS(my_destructor)
  #endif
  G_DEFINE_DESTRUCTOR(my_destructor)

  static void my_destructor (void)
  {
    ...