[WIP] Fix Python shebang lines.

This avoids using a hard-coded value for the shebang, as
previously discussed in bug #650763.

https://bugzilla.gnome.org/show_bug.cgi?id=658237
[WIP] win32: fix assert-msg-test
[WIP: this might be obsolete] fix out of tree build for MinGW/MSYS

When building with MinGW/MSYS with srcdir != builddir the build fails:
- to locate the generated .def files
- creating libglib-gdb.py
- creating libgobject-gdb.py

Solved this by explicitly instructing these files to be generated
in $(srcdir) for .def files [1] or $(builddir) otherwise.

https://bugzilla.gnome.org/show_bug.cgi?id=653167

[1] As the nmake makefiles need the .def files to be available in
the source tree when building from a source tarball, we need to
put them in $(srcdir), unfortunately.
Let AM_PATH_PYTHON do it's job...

...and abort configure if no suitable Python interpreter
is found instead of forcing a non-portable (and most likely
non-existent) value on ACTION-IF-NOT-FOUND.
Fix some typos
glib/gutils.c: move section blurb below includes
win32: improve docs, don't give the impression we still support non NT based Windows versions
win32: add deprecation warnings to gwin32.h

g_win32_get_package_installation_directory and
g_win32_get_package_installation_subdirectory have been deprecated
in favor of g_win32_get_package_installation_directory_of_module
since GLib 2.18.1 (see commit e55fca9533b7cc6ada62fc1f51cf899b1dce7d8e).
win32: remove unused include from gwin32.c
win32: define WINVER and _WIN32_WINNT to 0x0501 by default when building glib

This formally raises the minimum supported Windows version
to Windows XP.
win32: define STRICT by default when building glib
win32: we can safely assume GetModuleHandleEx is going to be available...

... as we now require at least Windows XP.
win32: remove unused fallback defines

- msvc 5 hasn't been supported for a while now.
- We require at least Windows XP, so it is safe to assume
whatever SDK used to build GLib will have these defines.
win32: merge G_PLATFORM_WIN32 specific includes
win32: remove no longer used includes
win32: rename private _glib_get_dll_directory() to _g_win32_get_glib_dll_directory()

and improve readability a bit.
win32: remove "windows directory root" fallbacks for g_tmp_dir and g_home_dir

g_tmp_dir: GetTempPath() behavior is documented to return one
           of %TMP%, %TEMP%, %USERPROFILE% or the Windows
           directory (C:\Windows).
g_home_dir: better alternatives are already searched (CSIDL_PROFILE
            and %USERPROFILE%, for example).

The "windows directory root" (C:\) is not a good choice for these
as there is a good chance a user will not have write permissions
for that location with newer NT class Windows versions.
win32: use GetTempPath for g_tmp_dir

%TMPDIR%, %TMP%, %TEMP% and get_windows_directory_root() were
checked in that order. However, on both XP an 7 %TMPDIR% does
not exist. Also, this behavior does not correspond with GetTempPath()
which returns one of %TMP%, %TEMP%, %USERPROFILE% or the Windows
directory (not the same as get_windows_directory_root()!), checked
in that order.

So simply use GetTempPath() instead.
win32: add private _g_win32_get_known_folder() function

_g_win32_get_known_folder() will replace the currently used
win32 specific get_special_folder implementation and the
mess in load_user_special_dirs().
win32: Windows documentation is usually written after the POSIX/Linux/UNIX specifics